FOR all the fun we have with them, illusions do serious work in illuminating how our brains work, and in particular how perception works. They may also help us understand how consciousness developed, and tell us about our “neuro-archaeology” and the behaviour patterns laid down in the nervous system over evolutionary time.

But let’s concentrate on perception: it is tricky enough. I’ve tried to classify illusions in a way that shows the principles underlying them, starting with physical causes, moving on to physiological disturbances of neural signals, and finally to cognitive processes – where the brain tries to make sense of sensory signals, not always successfully.

The distinction between physiological and cognitive is not straightforward. It’s rather like the distinction between how a machine works and what it does. For example, a can opener needs two descriptions: the mechanism of levers and cutters, and what this does to open a can.

That distinction between physiological and cognitive has “real-world” consequences. Think of the placebo effect, which suggests close connections between the physiological and the cognitive-psychological. So different types of illusions could be significant in ways we do not yet know. That’s why I have constructed my Peeriodic Table of Illusions (bad pun intended) thus: blindness, the ambiguities, instability, distortion, fiction, and paradox, plus their causes.

Starting with blindness perhaps seems odd but the many kinds of blindness and accompanying visual phenomena tell us much about perception. Blindness ranges from the physiological, with no sensation of light and colour (congenitally or from injury or disease) to various mind blindnesses, such as agnosia, when light, colour, movement and form are perceived but the object seen lacks meaning.

Another form is change blindness, where a person fails to notice big differences in a picture or scene – sometimes even when someone in that scene has been substituted.

Next come the ambiguities. Confounded ambiguity illusions depend on a failure to properly distinguish between two objects, in poor light or because of our ageing senses. Differences in the brightness of regions of an object or scene help us see detail; limited light makes the visual brain choose whether neural activity is due to the presence of light or to neural “noise”. Both neural noise and light fluctuate randomly so to see anything reliably we need significantly more photons.

As for flipping ambiguity, such as the duck-rabbit illusion, there are two theories about how they work: either the brain tires of one image and switches to the other, or there are two perceptions vying for centre stage. Since perception usually changes when what is “out there” changes, this spontaneous flipping may tell us the brain is switching its opinion as it ponders alternative interpretations. Oddly, flipping gets easier with practice. It is as if more or less feasible alternatives wait in the wings to challenge the present interpretation. Once, after weeks looking at ambiguous figures, I found solid objects, even concrete buildings, flipping in front of my eyes!

One of the most famous kinds of instability illusion is created by the use of those repeated patterns so typical of 1960s Op Art, which make the picture appear to be moving. Again, the causes are controversial. One view is that these patterns stimulate brain regions in the V5 area to produce sensations of movement. Or it may be that there is motion at the retina from eye tremor, and from the lens trying to focus the image, which may stimulate the movement systems – especially from high-contrast repeated lines.

Distortion illusions are arguably the most controversial since they most concern the distinction between illusions created by receiving neural signals (reception), where things can go wrong physiologically, and illusions of misreading signals (perception), where things can go wrong cognitively; back to the physiological versus the cognitive again.

Years ago I was struck by the idea that Ponzo illusions and the Muller-Lyer illusions, which normally show converging lines and arrows, are simple perspective pictures of 3D objects (corners, in the case of Muller-Lyer), or scenes (receding railway tracks, in the case of Ponzo illusions), and that to understand them we should think about how three dimensions are seen in 2D pictures. This includes the retinal images of normal objects.

We know what we see is very different from the images on our retinas because perceptions are scaled, like maps. So what sets the scaling for seeing the sizes and shapes of surrounding objects? Using ambiguity illusions I found that the scaling in Ponzo and Muller-Lyer illusions can be set from visual cues, such as the convergence of lines by perspective, or from the current perception of distance. The fact that the same retinal image can give more than one perception, as when perceptions “flip”, is useful because it lets us separate “bottom-up” (from the eye) from “top-down” (from the brain) processes. This way we know that a perceptual change without a change in the eye must be top-down, from the brain, and not bottom-up, as there is no change in the image.

This leaves us with fiction and paradox. Fictional illusions are not necessarily false, any more than a novel is altogether false, though fictional. They are generated by the creative activity of the visual brain, generally guided by knowledge from the past, often predicting the immediate future. There are probability-statistical principles here known as Bayesian inference.

One “fiction” concerns the blind spot on the retina, where the optic nerve is. One of nature’s most amazing illusions is that we don’t see this region as a black hole in visual space. The brain generally “fills in”, using surrounding colours and patterns.

“One of nature’s most amazing illusions is that we don’t see our blind spot”

The last category, paradox, brings us to René Magritte’s Carte Blanche (left). The illusion turns on the unlikeliness or impossibility of an event. For example, a person swimming the Atlantic is unlikely, but it is “allowed” by the rules of language; a dark-haired blonde, however, is impossible and not allowed by the rules, so it is a logical paradox. In Carte Blanche, we see an impossible horse we know could not be ridden or even be alive. Why? While the probable is by definition more likely to be occur, unlikely things do occur and we need to pay them special attention. But the Magritte horse? My hunch is that perceptions are hypotheses, depending both on rules, which may conflict, and on assumptions, which may be wrong.

As we have seen, there are a great variety of causes of these phenomena of seeing we call illusions. Many are imperfectly understood, and some have wildly different explanations. But illusions are invaluable because the clues they hold to how we see simply could not be found elsewhere.

SCIENTIFIC instruments are evocative at a philosophical level: they match the
senses, especially the eyes, to selected features of the physical world, making
visible the invisible. They allow precise measurement and superhuman
calculations—and keep philosophers employed debating the meaning of
“observation”. Other, less familiar, functions may stretch our minds yet more:
what of the “philosophical instruments” designed to demonstrate and to propagate
the theories which resulted from experiment?

Instruments have affected scientific theories from the Middle Ages to the
present. New theories have also set demands for new instruments. Devices that
could make accurate positional measurements in astronomy, pioneered by Tycho
Brahe, were especially important both for Newtonian physics and for the practice
of navigation. Here, of course, the development in the 18th century of the
seagoing chronometer by British clockmaker John Harrison was of prime
importance.

As Gerard Turner points out in Scientific Instruments 1500-1900,
instruments were not made only for research. Used in public demonstrations they,
no doubt, advertised the skill, and in some cases the wealth, of investigators
and teachers. They form an interface not only between the senses and the
physical world, but also between science and art. It is perhaps this that most
appeals to amateur collectors and visitors to science museums, for they are
indeed aesthetically beautiful as well as conceptually intriguing objects.

The wide range of instruments that Turner presents might be designed to make
amateur collectors jealous. He illuminates what they may be able to buy in
antique shops, and presents splendid examples beyond the means of most of us. Of
particular use to the collector is a table of the angles of declination of the
Earth’s magnetic field near London, from 1580 to 1910. The surprisingly wide
variations are helpful for dating some instruments, and so possibly for
eliminating forgeries.

Turner divides the instruments into a dozen groups, each beautifully
illustrated. There are instruments for astronomy and telling the time,
navigation, surveying, drawing and calculating, optics, weights and measures,
and medicine. Perhaps the most intriguing are the “philosophical instruments”.
Demonstrators used these to elucidate magnetism, pneumatics, hydrostatics and
hydraulics, electricity, heat, sound and light, and meteorology. They were
particularly useful for demonstrating the basic physics of motion, forces and
the effects of friction.

Friction was usually seen as a contaminant, polluting the beauty of the
frictionless world of astronomical bodies. Reducing friction to a minimum was
thought to reveal Newton’s universe in the study or the school room. Models of
steam engines showed how efficiency could be improved by understanding
thermodynamics and overcoming the effects of thermal inertia, which wasted
power. So the “philosophy” ranges from the most abstract to the most
practical.

We now see this as the essential basis of “hands-on” science centres. This
approach fortunately appeals greatly to children, and to the remaining childish
curiosity of adults. Curiosity perhaps sets us apart from all other animals,
placing us (at least in the eyes of scientists) close to the gods.

There is a great need for new “philosophical” instruments to be made
available in schools. New forms are needed to bring out the concepts of modern
physics, including quantum mechanics. They should be attractive and
aesthetically appealing, allowing children to extend their senses and minds to
appreciate current discoveries and questions for the future. The drama of the
history of science engendered by human curiosity is evident in historical
scientific instruments, which are beautifully illustrated and described in this
book. Perhaps they will inspire modern designs for our age.