senses, extension of There are two, essentially different, ways in which human beings can extend their sensory powers (neither of which involves extra-sensory perception!). Passive extensions do not need extra power; active extensions require some source of energy.

The most obvious way in which we can escape from the physical limitations of our eyes is to employ a microscope, magnifying glass, or optical telescope to improve magnification and resolution. It is remarkable that such passive optical devices have no running costs. One seems to get something for nothing. But this is not quite right. Whatever the properties of the optical system within and in front of the eye, its capacity to resolve patterns of light is ultimately limited by the spacing of the array of photoreceptors in the retina, which catch the light and initiate the signals that are sent to the brain. Consider the fact that coarse-grain film always produces grainy photographs, however excellent the camera in which it is used. Imagine an eye with only one photoreceptor: it would provide the brain with no information about the pattern of light and dark in the image, whatever the quality of its optical system.

Although external magnification can be very useful for particular purposes, there is no gain of information. A magnifying glass enlarges things at the cost of reducing the total field of view: so, one sees more of less. This is valuable for close examination of small objects, but if the optics of our eye were designed to provide such magnification, vision would be perpetually limited to a small fraction of the normal field of view. Evolution has generally provided animals with a wide field of vision to warn them of danger from all sides. Hence the resolving power of their retinal photoreceptors is distributed across a wide region of space, sacrificing resolution for coverage. (The enormous blind region behind our heads is not, of course, evidence against Darwin! For animals that hunt or manipulate things with their hands, there are other advantages in having eyes that point forwards, with their fields of vision overlapping — not least the capacity to judge distance by stereoscopic vision.)

Try walking around while viewing the world through binoculars or a magnifying glass to see how inconvenient it would be to sacrifice the field of view for the sake of greater resolution. The rear-view mirror in a car does, however, provide the all-round view that evolution has failed to achieve — a very useful extension of vision. However, it does it by making use of part of the eye that would normally be viewing the world ahead. Using such a device requires a good deal of practice.

The sensitivity of the photoreceptors is so great that the best of them (the rods, which provide us with vision in dim light) attain the theoretical maximum: they respond to a single quantum of light. Why, then, does an astronomical telescope allow stars to be seen that are far too faint for unaided sight? The large lens (or even larger curved mirror) captures more quanta than the small pupil of the eye, and hence increases the chance that enough of them will be caught by photoreceptors for the brain to see the star. Also, the eye can be replaced with a camera that integrates quanta over minutes or hours, unlike eyes, which have to signal rapidly to the brain if their information is not to be too late to be useful for most behaviour. (Actually, when the eye adapts to dim conditions, it does integrate for a longer period, allowing it to detect fainter targets, but suffers the disadvantage of not being able to signal rapidly changing or moving objects reliably. This results in the dramatic Pulfrich Illusion — the apparent circular swinging motion of a pendulum when viewed with a dark glass over one eye.)

Devices have been invented to give new uses to our senses, extending them beyond the biological function for which they evolved. But for such a device to be valuable, it has to be matched to the normal use and situation of the sense it extends. And extension is always at some cost, though this may not matter for the particular, limited purpose of the instrument.

Active extensions can be unimaginably dramatic in their effects, providing entirely new kinds of information for everyday living, as well as for science and technology. The unaided eye is sensitive to just one octave out of the vast spectrum of electromagnetic radiation that exists in the universe. Radio and television (involving the transmission and reception of electromagnetic radiation for which we have no sense organs) gives communication undreamed of a century ago, opening our minds to entirely unexpected features of the universe. X-rays, discovered in 1895 by the Austrian physicist Wilhelm Röntgen, were immediately seen as valuable for medical imaging. Diffraction patterns of the short wavelength, high-energy X-rays pointed to the structure of DNA, and X-rays have revealed the hidden structure and dynamics of matter itself. Essentially such techniques extend human sensitivity to regions of the electromagnetic spectrum, outside the bounds of the tiny fraction of that spectrum that we can see.

Extensions of damaged senses are one class of prostheses. Most common are spectacles, which are passive, and hearing aids. Electronic hearing aids are active, but their predecessors, ear trumpets, were passive. They worked by selecting the wanted source of sound, for example someone else speaking, while reducing surrounding auditory irrelevancies. This function is hard or impossible to achieve with inconspicuous, small electronic devices. Also, simple amplification can produce damage to the delicate hair cells of the cochlea. Vanity is the great obstacle to the design of hearing aids. The joke has it that the most effective aid is simply a piece of string dangling from the ear: this encourages others to speak more slowly and clearly — a psychologically active hearing aid!

Richard L. Gregory

See also ear, external; hearing; hearing aid; imaging techniques; microscopy; vision; X-rays.