Science Picture Quiz Archive

Week 29 Answer:

Why on Earth are people emptying floating, black balls into this lake? It's actually a clever solution to a very serious problem! 

This particular lake is the Ivanhoe reservoir in Los Angeles, which provides drinking water to the city. It is fed by water coming down from the mountains, which includes some saltwater, which carries bromide. Bromide is, in itself, harmless to people, but in certain conditions, it forms bromate, which causes cancer. Scientists at the Los Angeles Department of Water and Power unexpectedly discovered that the reservoir contained dangerous levels of bromate, but how come? 


It turned out that the bright sunlight was causing bromide molecules to react with the chlorine molecules used to disinfect the lake and this reaction made bromate molecules! It is virtually impossible to remove bromide from the reservoir and the chlorine was essential to make the water drinkable, so it was the sunlight that needed to go! And that's where 96 million black, plastic balls come in! They block the sunlight from entering the lake and so, prevent the reaction taking place.


The balls are, in fact, partially filled with water, so that they are not blown out the reservoir by the wind, and their colour comes from added carbon, so that it can withstand 10 years in the sun without fading! 



Credit: Irfan Khan/AP

Week 28 Answer:

So whose fossilised remains were we puzzling over this week and what is their scientific or technological significance? Pat yourself on the back if you managed to track down the 50-million-year-old Indohyus, the missing link in an evolutionary chain. But whose ancestor is it? You may have been surprised to find that Indohyus is believed to be the ancestor of...whales, dolphins and porpoises - the cetacean family! 

As long ago as Darwin, various clues had revealed to scientists that the cetacean family had in fact evolved from land-dwelling mammals. They have lungs and breathe air, give birth to live young and nurse them with milk. And if you look at their skeletons, you find that they have hand bones with five 'fingers' inside their large, front flippers, while whale embryos develop tiny back limbs, before losing them again before birth.


Cetaceans also have a thick covering of bone, called the involucrum, over the middle-ear space, that no other family of creatures has. This amplifies vibrations and is thought to help cetaceans hear underwater. Fossils, such as Ambulocetus - an amphibian that lived around 48 million years ago - also have this tell-tale bone, as well as thickened leg bones to weigh it down in the water, and so have been identified as ancestors of the cetaceans, showing an in-between step from land mammals to marine mammals. Now, scientists have found that Indohyus also has thick leg bones and an involucrum and so fills in an even earlier piece of the puzzle. And fascinatingly, isotope analysis of its teeth show that it ate land plants, not fish. This has lead scientists to conclude that the ancestors of the cetaceans took to the water to avoid predators, rather than to hunt fish. 




Week 27 Answer:

Well, it was a medical illustration of some sort, but did you manage to track down the scientific or technological significance of this week's picture? If you did, you'll know it is the discovery of the pulmonary circulation the mid-13th century! So what is the pulmonary circulation system? 

We are now familiar with the idea that the right side of our heart pumps blood to the lungs, where it picks up oxygen, before flowing into the left side of the heart, which then pumps it round the body. But at the time of this illustration, it was still believed that blood flowed from the right side of the heart into the left side of the heart through invisible pores in the septum, which is the dividing wall between the two sides, and only flowed to the lungs and back from the right side of the heart, in order to nourish the lungs. This belief was based on the theories of an Ancient Greek scholar, called Galen, who had lived a thousand years earlier, and had been accepted as fact all this time. It was a scholar from Damascus, called Ibn al-Nafis, who first challenged these accepted ideas, arguing from observation and common sense that blood could not pass through the thick, solid wall of the septum, but instead passed through the lungs. He also predicted the existence of pulmonary capillaries, tiny blood vessels joining the pulmonary artery and pulmonary vein, which were not observed for another 400 years. This week's picture is an illustration of the pulmonary circulation system from Ibn al-Nafis' work, Commentary on Anatomy in Avicenna's Canon, published when he was only 29 years old


Week 26 Answer:

This must easily be the oldest manmade image we've ever had on the picture quiz, but what is it, and what is its scientific or technological significance? Pat yourself on the back if you tracked down tablet YBC 7289 and the most accurately-calculated irrational number in the ancient world! 

This week's picture is of a clay tablet and its inscription, written about 3600-3800 years ago in southern Iraq during the Babylonian Empire! It shows the calculation of the length of the diagonal of a square of side length 30...which includes √2, calculated correctly to six decimal places! Whereas we write numbers in base ten - for example, 235 means (2 x 100) + (3 x 10) + (5 x 1) - the Babylonians also wrote in base ten up to 60, but then used a base 60 system. So √2 is written as:

1 24 51 10

which means:

1 + 24/60 + 51/3600 + 10/216000 

which is about 1.414213! 

By studying other clay tablets - there are estimated to be between about half a million and two million of them from Ancient Babylon -  historians know that Babylonian scribes had lists of important numbers, like we have a formula and  data booklet. Historians believe this calculation was probably written by a trainee scribe, learning to do the calculations they would need for their future career, who would have just looked up the value of and √2 used it. But from other mathematical tablets, scholars believe the method of calculating the value would have involved a geometric approximation. 


Babylonians wrote on the clay tablets in a script called cuneiform using a stylus, which was a stick with one end cut to a wedge shape. As well as maths and accounting, astronomical records and epic poetry have been found and translated! The Epic of Gilgamesh is considered the oldest surviving work of literature and was found on a tablet dating from about the same time as this one. 



Credit: Urcia, A., Yale Peabody Museum of Natural History


Week 25 Answer:

It was yet another grainy, black and white image this week, and as usual, there wasn't much to go on! So congratulations if you tracked down the surprising story of...the discovery of radioactivity! 

In 1896, Henri Becquerel was studying certain uranium salts that glowed, after they had been left out in the sunlight - a phenomenon called phosphorescence. He heard about the recent discovery of x-rays and wondered if that's what the uranium salts might be emitting, so he wrapped photographic plates in black paper and left them outside in the sunshine with uranium salt crystals on top. He reasoned that the black paper would block the sunlight, but if the uranium salt crystals absorbed sunlight and then emitted x-rays, there would be dark patches on the plates where the crystals had been. When he found dark patches on the developed plates, he concluded that he was right! Which just goes to show how human nature plays a role in the work of scientists - the dark patches showed that radiation of some sort was coming off the uranium salts, but it did not prove that this radiation was x-rays, nor that the uranium salts emitted radiation because they had absorbed sunlight! But is easy to jump to the wrong conclusions, when you really want to make an important discovery! Becquerel even reported his 'discovery' at a meeting of the French Academy of Science! 

Luckily for Becquerel, he went to do some more of these tests and one day, it was cloudy. He decided there wouldn't be enough sunlight to show up any outline, so instead, he put the wrapped plates with the uranium salts on top away in a drawer. When he took them out again, some time later, he luckily decided to develop the plates anyway, expecting to see no dark patch or perhaps only a very faint one, as the uranium salts had not been able to absorb any sunlight. However, what he got was the picture we have been looking at this week! This lead Becquerel to conclude, correctly, this time, that the radiation emitted by uranium salts was not caused by the uranium salts absorbing sunlight. Radioactivity had been discovered! 

And there you have it: an incredible true story showing the effects of emotions and chance on scientific progress! 


Week 24 Answer:

Congratulations if you tracked down Pilobolus crystallinus, common name: Hat Thrower or Dung Cannon! But why does this species of fungus merit an appearance in the Picture Quiz? 

Undoubtedly there is plenty of scientific significance to know about this little fungus, which is only about a centimetre high, but perhaps most impressive is that it achieves the greatest acceleration of any living thing! The black dots are the spores of the fungus and each one has a tiny balloon filled with cell sap underneath it. The fungus pumps more and more cell sap into the balloons until finally, the pressure is great enough to burst them. As the cell sap shoots out, it accelerates the spores up and away at an incredible 20,000g - that's 20,000 times acceleration due to gravity! 

It does this because the fungus - as its common name suggests - grows on animal dung, which it eats! To develop properly, its spores need to pass through the gut of a herbivore, such as a horse, but herbivores won't eat grass near dung. So the fungus needs to get its spores far enough away from their dung home to be eaten - out of the so-called "zone of repugnance"! The spores are so small and light that air resistance on them is large in relation to their mass, so even with these huge accelerations, they don't actually travel very far. But it's impressive to note  the comparison with a bulled fired from a rifle or shotgun, which doesn't even achieve an acceleration of 10,000g! The acceleration of the spores is so huge that, to the human eye, they appear to just disappear! Click the second link below to see for yourself! 



Credit: Sava Krstic

CC BY-SA 3.0

Week 23 Answer:

Yes, this week's picture is of a crocheted coral reef! But what on Earth is the scientific or technological significance of this?! Well it all begins with hyperbolic geometry... 


Space as we experience it is Euclidean, which means that the angles inside a triangle add up to 180 degrees. It also means that, for any given straight line and point, you can only draw one straight line through the point parallel to the first line, and the two lines will never meet. 


But imagine if you were drawing on the surfaces of a football. Then the angles inside a triangle would add up to more than 180 degrees and it would be impossible to draw two straight parallel lines that never met. And if you drew on a horse's saddle, the angles inside a triangle would add up to less than 180 degrees and you could draw multiple straight lines through a point parallel to a first straight line. 


A surface that has these properties of a horse's saddle at all points is called a hyperbolic plane, and that's where the crochet comes in! The number of stitches in a line in a piece of crochet can be increased exponentially row on row by, for example, adding an extra stitch after every fifth stitch. This quickly creates the crinkly structures of the corals, which can be shown to behave like the horse's saddle! 

Such hyperbolic structures are found elsewhere in nature - sea slugs also sport them! Their advantage is that they very quickly increase the surface area of a branch, which allows for a higher rate of mineral absorption or gas exchange, for example. 



Credit: Margaret Wertheim

CC BY 2.0

Week 22 Answer:

A bit of detective work was needed to track down this week's picture, but if you followed the clues, you'll know that this is the first illustration of the supernova SN 1572, but that its significance doesn't stop there! 

Star "I" in the picture is the "nova stella" - the new star - that appeared in the constellation of Cassiopeia on 11th November 1572. It was observed and recorded by the Danish astronomer, Tycho Brahe, who published this diagram in his 1573 book called De Nova Stella. But why was this discovery so important?


The Moon and the other planets change their positions against the backdrop of the fixed stars over days and months. The measurement of a body's position against the fixed stars is called its parallax. The more distant a body, the less its parallax changes. Brahe made careful measurements of the parallax of the new star and found that it did not change; the new star did not move relative to the fixed stars. And this was groundbreaking! At the time, European scholars believed that everything in the universe beyond the orbit of the Moon was eternally unchangeable. Brahe knew his observations proved the new star was further away than even the planets! The publication of his book made his name in European scientific circles. 

Brahe was also responsible for the data that led Kepler to formulate his laws of planetary motion. Apart from his achievements in astronomy, Brahe is notable for losing part of his nose, aged 20, in a duel with his cousin (over who was the better mathematician) and wore a prosthetic nose for the rest of his life! 


Week 21 Answer:

This has to be one of the  most challenging pictures we've ever had in the Picture Quiz, so congratulations if you worked out that this is a decimal calculator tool from Ancient China!


Almost 2000 years before Napier invented his 'Bones' (featured in a recent Picture Quiz) to speed up calculations, the Chinese used this matrix of bamboo strips to carry out decimal multiplication. The 21 strips, each over one metre long, all have the same 19 numbers down the right-hand side: 0.5, the integers 1 to 9 and the multiples of 10 from 10 to 90. The entries at the intersection of each row and column in the matrix provide the results of multiplying corresponding  numbers. Numbers not directly on the matrix can also be calculated, for example:

17.5 x 8.5 = (10+7+0.5) × (8+0.5) = 10 x 8 + 10 x 0.5 + 7 x 8 + 7 × 0.5 + 0.5 x 8 + 0.5 x 0.5 

like you would multiply a cubic polynomial by a quadratic.  

This makes the matrix of bamboo strips an ancient decimal calculator - it dates from 305 BC! The oldest multiplication tables are an incredible 4000 years old and were invented by the ancient Babylonians, but they are not base-10 but base-60!

Week 20 Answer:

What were the lines beside each element symbol and what was the significance of the step-like arrangement? Pat yourself on the back if you worked out that this week's picture was Moseley's staircase and that it lead to a crucial breakthrough in our understanding of the elements!


In 1913, Moseley was experimenting firing electrons at a metal target inside a vacuum tube. This caused x-rays to be emitted by the metal atoms in the target. By refracting these x-rays, Moseley separated them out into their different wavelengths, with shorter wavelengths refracting more. By putting a photographic material in the way, Moseley took a picture of the x-rays that showed bright lines corresponding to the wavelengths emitted, with the position of each line revealing the wavelength of the x-ray. A set of lines like this is called an emission spectrum. The black rectangles in this week's picture are the x-ray emission spectra Moseley photographed! 


But why the step arrangement? This is what you got when you placed the spectra in order of the elements' positions in the periodic table and with wavelength along the horizontal axis. At this point, Moseley could see that the dominant (brightest) x-ray line in each spectrum formed a neat and even staircase shape, so that moving from one element to the next element in the periodic table took you one step up the staircase. But why was this such a breakthrough? 


Because in 1913, protons and neutrons had not been discovered yet and the elements in the periodic table were grouped by similar chemical properties and ordered by increasing mass. The cause of the patterns was not known and puzzlingly, the masses of neighbouring elements were occasionally the wrong way round! What Moseley's staircase showed was that the order in the periodic table was correct: the fundamental difference between the atoms of different elements was not their masses. It was some other fundamental property that went up in neat steps from element to element in the periodic table. Soon after, protons were discovered and the fundamental property was revealed to be proton number!


Week 19 Answer:

Was it a game? Was it a measuring tool? No, it was a calculating tool! This week, we were looking at (a very old set of) Napier's Bones! So what was the significance of those? 

Napier's Bones, invented in 1617 by John Napier, were numbered rods that allowed large numbers to be multiplied and divided easily via a method that works just like how long multiplication is taught in schools today. (Click the top/bottom link below to read/watch Napier's bones in use!) They could also be used to find square and cube roots. Napier wrote that he invented this tool to avoid "the tedious expense of time" such calculations involve and also to combat the problem of the "many slippery errors" they are subject to, something I think we can all still identify with, four hundred years later! Napier's Bones were used to calculate accurately for hundreds of years. In 1891, a railway engineer, called Henri Genaillle, improved on them, but his version - Genaille's rods - was quickly superseded by the first mechanical calculators! 

John Napier was a brilliant Scottish scholar, best known for discovering logarithms. He was alive just as the Renaissance was getting started, a time when scientific methods were taking root, yet medieval beliefs were still widespread. Napier studied mathematics, physics, astronomy, agriculture and military technology, but also the occult and had a pet black cockerel, believed by many to be his familiar! 



Credit: Kim Traynor

CC BY-SA 3.0

Week 18 Answer:

Did you manage to track down the fossilised leaves in this week's picture? If you did, you will know that they belonged to Glossopteris trees, which lived roughly 300-250 million years ago, and that their particular significance lies in their distribution

Fossilised Glossopteris leaves are found on all the continents of the southern hemisphere. These continents have oceans between them and do not have identical species on them today. So how can we explain the same species being found on them hundreds of millions of years ago? Towards the end of the nineteenth century, Eduard Suess was the first person to suggest that these continents had once been part of the same continent, which he called Gondwana. 

While Suess was wrong about how Gondwanaland was separated into smaller continents - he though sea levels had risen and simply flooded low-lying parts of Gondwana - the existence of Gondwana is still accepted. In 1912, Alfred Wegener proposed continental drift theory, which argued that Gondwana broke up into pieces, and that those pieces are the modern continents. Evidence for this theory included the shape of the coasts of the modern continents, matching geology along the west coast of Africa and the east coast of South America, continuation of mountain chains and the distribution of certain fossilised animal and plant remains, including Glossopteris. Continental drift is now accepted, but scientists at the time were sceptical, as Wegener did not have a plausible explanation for how Gondwana broke up. (He thought that the spinning of the Earth had caused the break up, but this was readily shown to be inadequate). In 1919, Arthur Holmes conjectured instead that the continents are carried by the Earth's mantle, which was on the move in convection currents, caused by the heating effect of radioactive decay in the Earth's core. Further work lead to the acceptance of this idea and the full theory of plate tectonics was born!  

Gondwana went on the collide with other former continents to form Pangaea, the last supercontinent, 335 million years ago. After another 160 million years, Pangaea began to break up and move apart, with the continents eventually assuming the shapes and positions they have today. Glossopteris was named for the shape of its leaves; it means "tongue fern" in Greek! 



Credit: Daderot

CC0 1.0

Week 17 Answer:

This week, we were looking at a game-changing medical breakthrough from 1971; yes, it was the first ever CT scan!  

X-rays had been used by doctors to look inside the human body since the turn of the century and were very good at imaging bones, but normal x-rays of soft organs, like the brain, did not give a detailed enough image to be useful. To improve the contrast between a tumour and the rest of the brain, unfortunate patients had air injected into their spinal columns and were then rotated to allow the air to bubble up into the brain. Patients often vomited from the pain caused by this procedure. It was Godfrey Hounsfield, a lead electrical engineer at EMI, who worked out a solution, allowing painless, high-quality imaging of the brain and much more, earning him a share of the 1979 Nobel Prize for Physiology or Medicine. 

Hounsfield's CT scanner worked by treating the brain like a sliced loaf of bread. An x-ray was taken of each slice and the strength of the x-rays detected on the other side. What was in that slice of the brain determined how much of the x-rays were absorbed and how much got through. Then the scanner was rotated by 1 degree and the process repeated. This continued until the scanner had taken 180 sets of x-rays. Perhaps the cleverest part of Hounsfield's CT scanner was the mathematical algorithm that took all this information, modelled the brain as a block of small cubes and worked backwards to figure out how dense each cube was, and it took the best computers at the time 2.5 hours to compute it! The scan for this week's picture was carried out in a London hospital and then involved Hounsfield driving across London, with the data recorded on magnetic tapes, to use an EMI mainframe computer! But the scan - though grainy and only 80 pixels in each direction! - was successful; the brain tumour was clearly visible.


CT scans quickly became widespread and have remained a powerful diagnostic and imaging tool. Developments over the years mean that now CT scans can even tell us the type of crystal forming inside a joint, allowing quick and non-intrusive diagnosis of conditions like arthritis and gout!    



Credit: A. Maier et al.

CC BY-ND 4.0


Credit: Edmund S. Higgins

CC BY-ND 4.0

Week 16 Answer:

So what was the scientific or technological significance of these horse and rider photos? Congratulations if you have been reading about chronophotography this week! 


Like me, you may have been aware that the people in the very earliest photos had to keep very still for several seconds while the photo was taken. During this time, the shutter stayed open, so that enough light could enter the camera to make a clear picture. A video, of course, requires many clear pictures to be taken in succession, so each picture has to form in a very short period of time. So how did we get from one to the other? 

A whole string of technological innovations were required, to make the coating on the photographic plates react quickly enough to light hitting them. In 1978, Charles Bennett published his process for "heat ripening" gelatin plates, which were sensitive enough to give effectively instantaneous photos, and this allowed the widespread taking of instantaneous photos for the first time. A photographer called Eadweard Muybridge was the first person to take a series of instantaneous photos of a moving object, by setting up a row of cameras with electromagnetic shutters triggered by tripwires; the object was, of course, this horse and rider! This week's picture is called "The Horse in Motion" and is the first example of chronophotography - photography used to record the passing of time - and so represents an important step on the way towards video! 


Muybridge was one of a group of photographers that used this new technology to study motion of living things. "The Horse in Motion" caused great surprise, as it had previously been believed that a horse always has at least one foot on the ground, as it runs!  


Week 15 Answer:

This week's picture was, of course, sketches of the Moon, but what was the significance of these particular sketches? They are some of the first pictures of celestial objects drawn using the aid of a telescope! They are also from the first book to publish such pictures; in other words, they are the pictures that revealed what the Moon really looked like to the unsuspecting world

The book, by Galileo, was called, rather fancifully, Sidereus Nuncius, which translates as "Starry Message", and was published in 1610. As well as pictures of the Moon, it contained pictures of stars that are not visible with the naked eye and the moons of Jupiter, which just look like points of light to the naked eye. Before this, people thought that the Moon was a perfectly smooth sphere and everything orbited the Earth. 


These sketches are NOT, however, the first sketches of the Moon made using a telescope, nor was Galileo the first person to see the Moon with a telescope, and the whole story serves as a nice reminder that scientists are rarely, if ever, working in splendid isolation! The first definite evidence of a telescope appears in the Netherlands in 1608 (when the first patent for one was submitted), but the properties of convex lenses were described by Ibn Sahl in his great work on optics in 984! Spectacles with two lenses to aid eyesight were in use from the 13th century. It was a spectacle-maker, Johann Lippershey, who applied for the first telescope patent. it is not clear who first put two convex lenses in a line and observed the effect, but many people may have known about it! An English astronomer and mapmaker, called Thomas Harriot drew the first known pictures of the Moon using a telescope several months before Galileo is known to have drawn his, (but he had reasons for wanting to keep a low profile - see last link below!) 

So does Galileo deserve the fame for Sidereus Nuncius and his Moon sketches? Almost certainly yes: Galileo made his telescope with significantly greater magnification than the Dutch telescopes at the time, from lenses he had ground himself, and he didn't just sketch the Moon. He also correctly deduced, from whether the edge of the shadow was jagged or smooth, that some parts of the Moon's surface were mountainous, while others were flat plains, and even used the shadows to calculate a reasonably accurate estimate for the height of the tallest mountains! He also described and explained the phases of the Moon in a very beautiful way! (Read it here!) 


Week 14 Answer:

Well it might not look like much, but if you managed to track it down, you'll know that it is iconic! This week, we have been looking at a moment in the development of Darwin's theory of evolution: this is the first time Charles Darwin committed his idea of the "tree of life" to paper, under the tentative note, "I think"! 


In The Origin of the Species, the book in which he published his theory of evolution, and from then on, Darwin used the analogy of a branching tree to describe how one species could develop into many different species. Each branching occurs when populations living in different environments evolve differently, as a result of the different pressures from their environments. Over millions of years, repeated branching leads to the large differences between species we see today. The branching tree analogy has been used to help explain evolution for over a hundred years. However...

...amazingly, modern geneticists now know that the "tree of life" model for how species evolve is too simple; in fact, branches do not always remain separate! Interbreeding between species is now known to be a lot more common than previously thought and even occurs in animals and plants, not just microbes! When fertile offspring result, this adds a 'tangle' between the branches, with chunks of DNA from one species appearing in the genome of another, complicating the relationships. Incredibly, some scientists in the field estimate that 10% of species regularly form hybrids with other species! 


Week 13 Answer:

Happy New Year and welcome back to the Science Picture Quiz! Over the holiday, you have hopefully been enjoying the amazing behaviour of...the Miura fold! Invented by the Japanese astrophysicist, Kōryō Miura, this famous origami fold allows a large, flat, rigid object to be folded into a much smaller, flat, rigid object! A neat trick, but what is the scientific or technological significance? 

The really neat thing about this fold is that a flat object folded in this way can be packed into a very small space and then unfolded or folded in one continuous action, making it ideal for deploying rigid structures like solar panels. The large solar arrays of a Japanese research spacecraft called the Space Flyer Unit were packed using the Miura fold before launch, so that they could be deployed easily once in space. Versions of the fold are also seen in insect wings and leaves before they open. Some scientists are researching ways to use the fold to make other shapes, such as tubes that could be inserted into a person's body while folded and 'popped up' into their unfolded shapes once in a blocked artery, to act as a stent, for example. The Miura fold may be the key to a generation of pop-up objects, giving new meaning to the term "flat pack furniture", and could even be used with sheets of graphene to create very small or very irregular shaped objects! 



Credit: Dmcq

CC BY-SA 3.0 


Credit: MetaNest

CC BY-SA 3.0 

Week 12 Answer:

It looks like it might just be somebody's doodle, but this week's picture was far from a mindless scribble! Congratulations if you tracked down Santiago Ramon y Cajal's original ink drawings of neurons, which led him to formulate the "neuron doctrine". So what was that? 

In the 1880s, when Cajal began his scientific career, the majority view in the scientific community was that the brain contained a single network of nerve fibres. A decade earlier, Camillo Golgi had developed a stain that allowed you to dye cells, so that when you looked at them under the microscope, you could see their internal structures better. Although still inconclusive, these better pictures of the nerve network in the brain suggested that the network might be made of separate nerve cells. Still better pictures were needed!  

By using thicker slices of tissue, a darker stain and embryological neurons (which have not grown their myelin shethes yet), Cajal managed to get clearer images of the nerve network under the microscope. As the technology to photograph them had not been invented yet, Cajal then meticulously drew what he saw, leading to the delicate picture on the right! From his observations, Cajal was able to prove that the network in the brain is made up of separate nerve cells - the neuron doctrine! The breakthrough earned him the 1906 Nobel Prize for Physiology or Medicine, which he shared with Golgi. 

So how did a talented scientist end up also being a talented artist? It turns out that Cajal always wanted to be an artist, but his father would not allow him to! He nonetheless continued practising in secret throughout his childhood and his artistic skills did indeed make an impact later on! 


Week 11 Answer:

Whose beautiful shell were we looking at this week and what was its scientific or technological significance? Your sleuthing hopefully led you to Roman togas, because this week's picture was of a Murex sea snail! 


This particular snail, from the Muricidae, or Murex, family lives in parts of the Indian and Pacific Oceans, but several species of snail from this family have been important for centuries, thanks to the dye they secrete. Believe it or not, the secretions from a Murex snail are the dye that gives us Tyrian purple, the classic deep purple familiar from pictures of Roman togas! Because it took thousands of snails and hours of labour to  dye a garment Tyrian purple - perhaps as many as 12,000 to dye the trim on a single toga - it was extremely expensive and came to signify high status. In Rome in the 4th century CE, it even became law that only the emperor was allowed to wear Tyrian purple. 

The dye itself is an organobromine compound, which is common in algae and some other sea life, but rare in land animals. Murex snails secrete the dye to when attacked, as a defence mechanism, but also use it to sedate prey and to protect their eggs, as it has antimicrobial properties. It can be collected by crushing the snails, but also by "milking" them, i.e. poking them to make them secrete the dye! This method is sustainable and obviously kinder to the snails, but a lot more labour intensive! 



Credit: Bricktop

CC BY-SA 3.0

Week 10 Answer:

So what was under the scanning electron microscope this week? Congratulations if you tracked down the lotus leaf, and its scientific and technological significance! In short, you cannot get a lotus leaf wet!  


If you have ever seen a lotus leaf, you may know that if you pour water on one, the water will simply sit in droplets on the leaf's surface and as soon as the leaf is tipped up, the water will run off. You may also have noticed that lotus leaves are always very clean. Why is this the case?


The upper side of a lotus leaf is covered in pointy mounds, only 10-15 μm across; these are what we can see in the photo. Water droplets and dirt particles are 'propped up' on the spikes and do not fall into the gaps. When the leaf is tipped, the water droplets roll off, picking up any dirt particles as they go. Dirt particles are too big to fall into the gaps, but water molecules aren't. So why are water droplets propped up like this?


It is partly because water surface tension is pulling the water together and a sphere is the 'most pulled together' shape, because it has the least surface area for its volume. The other part of the explanation is that the pull from the surface is low, in other words, the surface does not stick to the water well. This is because the spikes greatly reduce the area of leaf touching the water droplet and if that was not enough, these spikes are coated in wax tubes, less than a micrometer in diameter, in effect adding spikes to the spikes! So the droplets are touching an extremely small area of leaf and that area is made of wax, which is extremely hydrophobic (bad at attracting water)! This means the force attracting the water to the leaf's surface is lower than the force pulling the water into a ball, so the droplets keep their shape and do not stick to the leaf. 

Mimicking this surface structure has allowed engineers to make windows that don't need cleaning, as any dirt that does stick is washed off in the rain! Similarly, coatings have been developed to prevent algae and mould growing on outside surfaces, and NASA has developed a coating for spacecraft that repels dust. This technology also promises benefits in the food production industry, where containers have to be cleaned regularly. With a 'lotus leaf' surface, containers would not need to be cleaned as regularly, as fewer bacteria would grow on them, and could also be cleaned more easily, cutting energy usage and reducing the need for chemical detergents. 



Credit: American Chemical Society

CC BY 4.0

Week 9 Answer:

This week's unassuming but sobering graphs proved the cause of a life-threatening disease and so, told us how to avoid it. And the disease was: melanoma, the most serious type of skin cancer. 


Published in 1956, these graphs show the different rates of deaths due to melanomas in Australia, and England and Wales. This demonstrated conclusively for the first time that latitude - and therefore intensity of sunlight - was an environmental trigger of melanoma. While the link between sunlight and other forms of skin cancer had been easy to spot, the link between sunlight and melanoma was not obvious, as melanomas can occur on skin that is not exposed to sunlight and even on internal organs. Henry Lancaster, the author of these graphs, was in fact a mathematician and so this important medical development shows the role statistical analysis can play in scientific discovery. 

Not all light is visible and different wavelengths of light have other different properties too. We now know that ultraviolet (UV) light is responsible for melanomas and virtually all other skin cancers. UV light (wavelength ≈ 100 to 380 nm) is very good at ionising atoms it hits and this means it causes a lot of damage to the DNA in our skin cells, if it hits our skin. When DNA in skin cells is damaged, the instructions for how our cells copy themselves are messed up. This can lead to cancerous growths. 

The Earth's atmosphere blocks a lot of the UV arriving from the Sun, but not all, and the closer you are to the equator, the more gets through. Fortunately, wearing sunscreen or simply covering up in the sun is enough to protect us from the danger.  


Credit: H. O. Lancaster

Week 8 Answer:

Pat yourself on the back if you recognised this as a cloud/bubble chamber picture showing the tracks of subatomic particles! But what was the significance of this particular picture? 

This picture - of tracks in a hydrogen bubble chamber - was a significant step in the history of our study of the elusive neutrino! The existence of the neutrino was first hypothesised by Fermi in the 1930s to account for missing energy in beta decay reactions, but because it was thought to have zero mass and zero charge, it was not known how to detect it at the time. 


The first proof of the existence of neutrinos came in 1956, when Cowan and Reines detected the distinctive gamma rays given off when antineutrinos coming out a nuclear reactor hit protons in a tank of water. The gamma rays were detected by scintillators around the sides of the tank. 

This week's picture, however, is the first time tracks from a neutrino interaction were ever observed, allowing neutrino properties to be studied! In this interaction, a neutrino interacts with a proton, and the resulting tracks are the three lines that start together near the right of the picture and point off towards the left. The interaction causes the proton to shoot off (most nearly vertical track), while the neutrino disappears and a mu meson (long track to top left corner) and a pi meson (lowest track) are produced. 


This picture was taken inside a bubble chamber, rather than a cloud chamber, though these work on similar principles. A cloud chamber is filled with a supersaturated gas - for example, air supersaturated with alcohol. When a charged particle passes through, it ionises gas molecules. The alcohol condenses around these ionised molecules, leaving a visible trail of droplets where the original charged particle has been. A bubble chamber meanwhile, is filled with a pressurised, transparent liquid - usually liquid hydrogen - that is heated to just below its boiling point. This time, when the charged particle is about to pass through, the pressure in the liquid is suddenly dropped, which lowers the boiling point of the liquid. Now, the liquid is now superheated - this means it is still a liquid, even though it is now above its boiling point! Now when the charged particle leaves a trail of ionised liquid molecules, these evaporate, leaving a trail of bubbles instead. 

The bubble chamber, however, has pretty much had its day, as newer detectors allow the tracks to be saved digitally, allowing for easier analysis on computers! 



Credit: Argonne National Laboratory

Week 7 Answer:

Hopefully the box with the plates in it and two wires coming out gave you a clue as to the significance of our spooky, Hallowe'en picture! This cartoon is entitled, "The galvanised corpse", and shows the fear some people had of the emerging scientific phenomenon of... ...electricity! 

In the early days of the scientific study of electricity, scientists could store the charge from a lightning strike in a Leyden jar and they could generate static electricity in the lab, but they couldn't create electric currents, as no one had invented a battery yet. They also knew that some animals generated electrical currents and that animal muscles twitched, when touched by brass electrodes. The electricity in animals and the electricity being generated in the lab were shown to be the same thing by Alessandro Volta, who invented the first battery (called a voltaic pile) and used it to make frogs' legs twitch! 

Once this had been established, some scientists began using electricity to make newly-deceased human bodies twitch too! In 1803, Giovanni Aldini gave a famous public demonstration in London, in which he applied an electric current to the corpse of a freshly executed criminal, George Foster! This is how a publication at the time described what happened: 

On the first application of the process to the face, the jaws of the deceased criminal began to quiver, and the adjoining muscles were horribly contorted, and one eye was actually opened. In the subsequent part of the process the right hand was raised and clenched, and the legs and thighs were set in motion.

Galvanism demonstrations such as these made some fear we would soon be able to bring the dead back to life - hence this cartoon - and were the inspiration for Mary Shelley's Frankenstein! 


Week 6 Answer:

Well, it was something to do with magnetism and a ridge that is probably in Iceland, but did you get to the bottom of the significance of this week's picture? If you did, you will know that it provided the first evidence for seafloor spreading and ultimately helped prove the theory of Plate Tectonics!


Today, the idea that the continents rest on moving 'plates' is, not only accepted but, quite normal, but in the 1950s, most scientists thought it was ridiculous! So what changed their minds? Several pieces of evidence played a part and one of those was evidence for seafloor spreading. This is what is happening where neighbouring plates are moving apart; magma leaks out the gap between the plates, forming new rock when it cools. 

Our picture was taken with a fluxgate magnetometer, which is (not a prop from Star Trek, but!) an instrument that can very precisely measure magnetic field strength. It shows a section of the Mid-Atlantic Ridge that lies under water off the South-West coast of Iceland. The rocks in this ridge are magnetic; this is because they contain iron-rich minerals and, like little magnets, these iron-rich particles lined up with the Earth's magnetic field, while the rock was molten and then were fixed in this alignment, when the magma cooled. But what the black and white stripes show is that the direction of the magnetic field flips from band to band! So this suggests the Earth's magnetic field has also flipped direction repeatedly over long periods of time. 

And the real kicker is that the thickness and strength of the bands is symmetrical about the ridge itself, (which is shown by the thin black line through the central black band)! This is what you would expect, if rock was being continually formed along the ridge and then carried away from the ridge, as the two plates continued to move further apart. 

The first fluxgate magmetometer was invented during the Second World War for spotting submerged submarines. It could do this because large metal objects interact with the Earth's magnetic field, causing very small but measurable changes in the magnetic field strength detected by the magnetometer. After the war, magnetometers were put into service by oceanographers to map the magnetic field of the ocean floor, and that's when this key evidence for seafloor spreading was discovered! 



Credit: F. J. Vine, 1966

Week 5 Answer:

We've had Picture Quiz pictures that looked like magic before, but this week's surely takes first prize in that department! It is a photograph, which means it is not a hallucination! So how on Earth can we explain why this tanker appears to be sailing through the air? The significance of this week's picture is that it shows a superior mirage

Light may always travel at 300 million metres per second in a vacuum, but in materials, it travels slower. The more dense a material, the slower light travels through it, and vice versa. Warm air is less dense than cold air. So if light is travelling through air that is getting warmer, it speeds up. 

If the light is travelling at an angle through the material, one side of the wavefront is speeding up before the other side, which makes the light veer to one side. When light changes direction for this reason, we say it has refracted.


Very occasionally, the air directly above the ocean is colder than the air higher up. So light bouncing off a tanker does not travel in a straight line from the tanker to you - it bends downwards - and this is what happened to the tanker in our photograph when it was sailing near Falmouth in Cornwall in March this year! The tanker was actually just below the horizon, so the photographer wouldn't have been able to see it, if the light bouncing off it were travelling in a straight line. But the light bent downwards - round the Earth - into the photographer's eyes and camera! The photograph simply recorded where the light arrived, just as your brain always forms images based on where the light hits the retina, but your brain interprets images as though light travelled in straight lines. So you see the ship higher than its real position! In this picture, everything is just right, so that you can't see the actual water the tanker is sailing through, so the tanker appears to be sailing through the air! 



Credit: David Morris/APEX via BBC 

Week 4 Answer:

Did you manage to track down this week's mystery fossil? If you did, you will know that it is a beautifully preserved example of Dickinsonia. But what exactly was that and why is it significant? 


Dickinsonia was a soft-bodied sea creature belonging to the Ediacarans biota, which is the group of the very first macroscopic complex organisms in the geological record! This particular Dickinsonia lived 558 million years ago in Russia, putting it just before the so-called Cambrian explosion, in which the variety of animal species diversified massively and most of the major animal groups appear in the fossil record for the first time. Incredibly, this fossil is so well-preserved that even organic matter has survived! And this lead to a discovery that has made Dickinsonia even more significant! 


Scientists have found cholesterol in the fossil and this means Dickinsonia is in fact the very first known animal! The reasons scientists are so confident in the new classification is that lichens, bacteria and amoebas contain very low levels of cholesterol-like molecules, and this example of Dickinsonia contained very high levels up to 93%, compared with 11% in the surrounding sediment. Fungi do contain cholesterol-like molecules, but they also leave other stable molecules behind, that have not been found in Dickinsonia. 

Resembling perhaps a jellyfish or marine worm, sometimes over a metre long, Dickinsonia looked very different to even the animals that appear in the Cambrian explosion, raising interesting questions about how one might have evolved into the others.   



Credit: Ilya Bobrovskiy via BBC 

Week 3 Answer:

So what on Earth was the scientific or technological significance of last week's slightly macabre picture? Congratulations if you tracked down the role of the Pfeilstorch in proving bird migration! 

Pfeilstorch is a German word that translates as "arrow stork" and refers to a stork with an arrow stuck in it. Why did the Germans feel the need for such a word? Because this particular Pfeilstorch showed up in Germany in 1822, with a 30-inch spear from central Africa in its neck! And therein lies the scientific significance... 

Until this time, Europeans did not agree on what happened to certain birds, such as storks and barn swallows, in winter, when they were never seen in Europe. It was known that some birds migrated to warmer places over winter, but for other species, including storks and barn swallows, it was widely believed that they hibernated underwater! When Aristotle first proposed this theory 2,000 years earlier, the known world at the time for Europeans did not include the places these birds were overwintering. Incredibly, the theory persisted, even as we became aware of the rest of the world! There were, however, competing theories. Some people believed disappearing birds turned into other animals, while in 1703, one Harvard professor even wrote that these birds flew to the Moon!

It was the identification of the spear in this Pfeilstorch as African that proved the resilient bird had flown from a different continent to Europe and finally lead to the widespread acceptance that these disappearing birds did also migrate.  And it's incredible to note that it is 2,000 miles from its overwintering grounds to Germany! 



Credit: Zoologische Sammlung der Universität Rostock

CC BY-SA 3.0

Week 2 Answer:

Did you track down the orangey-yellow powder in last week's picture? If you did, you will know that it is gamboge, the pigment traditionally used to dye the robes of Buddhist monks. But what is its scientific or technological significance? It actually had a role in the final proof of the existence of atoms! 

It turns out that it was gamboge that was used to prove Albert Einstein's explanation of Brownian motion. Brownian motion is the random, zig zag motion that certain particles are observed to have, when they are in a fluid. Smoke particles in air and pollen grains in water exhibit Brownian motion. First described by the botanist, Robert Brown, in 1827, it wasn't explained until 1905, when Einstein suggested that pollen grains were being hit by individual water molecules that were too tiny to see, but moving very quickly! 

In 1908, Jean Perrin tracked the paths of particles under the microscope and proved Einstein's explanation. Perrin needed his particles to be of the right size - Brownian motion in water is observed with microparticles, which are between 1 and 1000 micrometres across - and also of uniform size, and that's where gamboge comes in! Gamboge, the pigment, is congealed latex from the gamboge tree. Latex is the substance a plant exudes as part of its defense mechanism when it is cut into, and is collected by tapping the tree. Chemically-speaking, latex is a "stable emulsion of polymer microparticles in water", which roughly means that it is a mixture containing long chains made from microparticles. Perrin devised a way to chemically separate the individual microparticles and then sort them by size in a centrifuge.  Then, by tracking the paths of individual particles seen through a microscope and using measurements of each path, Perrin showed that they gave the same value of Avogadro's constant each time, as predicted by Einstein's explanation!



Credit: Maša Sinreih in Valentina Vivod

CC BY-SA 3.0

Week 1 Answer:

So what was the significance of last week's colourful, high-altitude picture? It is the first ever colour photograph of an angel sprite! So what is one of those?

An angel sprite is actually a type of lightning! Lightning is an electric discharge from clouds that causes a flash of light. In a thunderstorm, tiny ice crystals and larger hail particles inside clouds rub against each other, rubbing electrons off the ice crystals and onto the hail.  This gives the ice crystals a positive static charge and the hail a negative static charge. The smaller crystals are carried upwards and the larger particles downwards, resulting in the build-up of opposite charges at the top and bottom of a cloud. When the bottom of the cloud discharges to the ground, it heats the air to such a high temperature, it turns into plasma, which glows. This is the 'normal' lightning that we see. 

This familiar discharge from the bottom of a cloud to the ground seems to sometimes cause a discharge from the top of the cloud up into the upper atmosphere. This excites the nitrogen atoms there; this means that the electrons in the atoms gain energy. When they lose the energy again, they do so by emitting red light, which we see as a sprite. This means that a sprite emits light the same way as a fluorescent light bulb! 

This picture was taken by geophysicist Davis D. Sentman in 1994, confirming reports of sprites from as early as 1886. 'Sprite' stands for "Stratospheric/Mesospheric Perturbations Resulting from Intense Thunderstorm Electrification" and a sprite often occurs at the same time as 'Elves', which are another type of lightning and whose name stands for "Emissions of Light and VLF perturbations from EMP events"! 



Credit: D.D. Sentman, Univ. of Alaska Fairbanks

Week 37 Answer:

Welcome back! Over summer we were looking at these three lovely Beluga whales and while they are undoubtedly scientifically significant for lots of reasons, the detail I'm hoping you spotted is the bubble rings they have blown! 

These are vortex rings, which means that the air inside them is spinning! The air is flowing up the inside of the bubble and down the outside in tiny loops all the way around the ring, and the water at the surface of the bubble is flowing round as well. Usually, the surface tension at the surface of a bubble acts to distort the bubble and quickly pulls it apart into smaller bubbles. But in a vortex ring, the flow of the water at the bubble's surface stabilises the bubble, by counteracting the effects of the surface tension. 

Incredibly, as a bubble spins, the spinning also generates a downward lift force, which partially cancels out the upthrust force and reduces the bubble's upward acceleration. However, it also causes the ring's diameter to increase. Even though the volume of the bubble is also increasing, (as the outside water pressure decreases), this enlarging of the diameter of the ring reduces its thickness, eventually allowing surface tension to break the bubble up. 

Dolphins have been observed playing with vortex rings they have made, pulling them around and biting them to burst them into smaller bubbles! 



Credit: Pagemoral

CC BY-SA 3.0

Week 36 Answer:

So what were the mystery microstructures in last week's picture? Congratulations if you tracked down fish lamellae! 

These structures are found in the gills of fish and the branches vastly increase their surface area. Why? Because this is where oxygen from the water diffuses into the fishes blood stream and carbon dioxide diffuses out! In other words, this is where gas exchange happens in the fish!


Oxygen and carbon dioxide diffuse into/out of the cells at the surface of the lamellae; this means the gases pass automatically from an area where they are concentrated to an area where their concentration is lower. The branches you can see here are only one cell thick, meaning almost the entire surface of a branch is available as an "entry/exit point" for oxygen/carbon dioxide. The lamellae are full of capillaries, so the gases get straight into/out of the bloodstream from there.  

The lamellae are very efficient and can extract 80% of oxygen from the water flowing over them in normal conditions. This efficiency results partly from the fact that blood flows through the capillaries in the opposite direction to the flow of water over the gills. This means that the water is most depleted in oxygen when the blood is too, ensuring what oxygen is left in the water still diffuses into the blood. 

Interestingly, all fish have the same sized lamellae, regardless of the size of the fish itself. This shows that the size and shape of the lamellae has evolved to maximise uptake of oxygen, rather than any other evolutionary pressure. 



Credit: Gtkwan
CC BY-SA 4.0