Eye, Brain, and Vision
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Right; The energy in a beam of light such as sunlight contains a broad distribution ot wavelengths, from 400 or less to about 700 nanometers. The gentle peak is a function of the temperature of the source: the hotter the source the more the peak is displaced towards the blue, or short-wavelength, end. Ri^lit: Monochromatic light is light whose energy is mostly at or near one wavelength. It can be produced with various kinds of filters, with a spectroscope containing a prism or a grating, or with a laser.




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                       COLOR VISION

The hundreds of dollars extra that consumers are willing to pay for color TV in preference to black and white must mean that we take our color sense seriously. A complex apparatus in the eye and brain can discriminate the differences in wavelength content of the things we see, and the advantages of this ability to our ancestors are easy to imagine. One advantage must surely have been the ability to defeat the attempts of other animals to camouflage themselves: it is much harder for a hunted animal to blend in with the surroundings if its predator can discriminate the wavelength as well as the intensity of light.
Color must also be important in finding plant food: a bright red berry standing out against green foliage is easily found by a monkey, to his obvious advantage and presumably to the plant's, since the seeds pass unharmed through the monkey's digestive tract and are widely scattered. In some animals color is important in reproduction; examples include the bright red coloration of the perineal region of macaque monkeys and the marvelous plumage of many male birds.
In humans, evolutionary pressure to preserve or improve color vision would seem to be relaxing, at least to judge from the 7 or 8 percent of human males who are relatively or completely deficient in color vision but who seem to get along quite well, with their deficit often undiagnosed for years, only to be picked up when they run through red lights. Even those of us who have normal color vision can fully enjoy black-and-white movies, some of which are artistically the best ever made. As I will discuss later, we are all color-blind in dim light.
Among vertebrates, color sense occurs sporadically, probably having been downgraded or even lost and then reinvented many times in the course of evolution. Mammals with poor color vision or none at all include mice, rats, rabbits, cats, dogs, and a species of monkey, the nocturnal owl monkey.
Ground squirrels and primates, including humans, apes, and most old world monkeys, all have well-developed color vision. Nocturnal animals whose vision is specialized for dim light seldom have good color vision, which suggests that color discrimination and capabilities for handling dim light are somehow not compatible. Among lower vertebrates, color vision is well developed in many species of fish and birds but is probably absent or poorly developed in reptiles and amphibia. Many insects, including flies and bees, have color vision. We do not know the exact color-handling capabilities of the overwhelming majority of animal species, perhaps because behavioral or physiological tests for color vision are not easy to do.
The subject of color vision, out of all proportion to its biologic importance to man, has occupied an amazing array of brilliant minds, including Newton, Goethe (whose strength seems not to have been science), and Helmholtz. Nevertheless color is still often poorly understood even by artists, physicists, and biologists. The problem starts in childhood, when we are given our first box of paints and then told that yellow, blue, and red are the primary colors and that yellow plus blue equals green. Most of us are then surprised when, in apparent contradiction of that experience, we shine a yellow spot and a blue spot on a screen with a pair of slide projectors, overlap them, and see in the overlapping region a beautiful snow white. The result of mixing paints is mainly a matter of physics; mixing light beams is mainly biology.
In thinking about color, it is useful to keep separate in our minds these different components: physics and biology. The physics that we need to know is limited to a few facts about light waves. The biology consists of psychophysics, a discipline concerned with examining our capabilities as instruments for detecting information from the outside world, and physiology, which examines the detecting instrument, our visual system, by looking inside it to learn how it works. We know a lot about the physics and psychophysics of color, but the physiology is still in a relatively primitive state, largely because the necessary tools have been available for only a few decades.



                                  THE NATURE OF LIGHT
Light consists of particles called photons, each one of which can be regarded as a packet of electromagnetic waves. For a beam of electromagnetic energy to be light, and not X-rays or radio waves, is a matter of the wavelength—the distance from one wave crest to the next—and in the case of light this distance is about 5 X 10 to the -7 meters, or 0.0005 millimeter, or 0.5 micrometer, or 500 nanometers.
Light is defined as what we can see. Our eyes can detect electromagnetic energy at wavelengths between 400 and 700 nanometers. Most light reaching

 

 

 

 

 

 



our eyes consists of a relatively even mixture of energy at different wavelengths and is loosely called white light. To assess the wavelength content of a beam of light we measure how much light energy it contains in each of a series of small intervals, for example, between 400 and 410 nanometers, between 410 and 420 nanometers, and so on, and then draw a graph of energy against wavelength. For light coming from the sun, the graph looks like the left illustration on this page. The shape of the curve is broad and smooth, with no very sudden ups or downs, just a gentle peak around 600 nanometers. Such a broad curve is typical for an incandescent source. The position of the peak depends on the source's temperature: the graph for the sun has its peak around 600 nanometers; for a star hotter than our sun, it would have its peak displaced toward the shorter wavelengths—toward the blue end of the spectrum, or the left in the graph—indicating that a higher proportion of the light is of shorter wavelength. (The artist's idea that reds, oranges, and yellows are warm colors and that blues and greens are cold is related to our emotions and associations, and has nothing to do with the spectral content of incandescent light as related to temperature, or what the physicists call color temperature.)
If by some means we filter white light so as to remove everything but a narrow band of wavelengths, the resulting light is termed monochromatic (see the graph at the right on this page).


                                           PIGMENTS
When light hits an object, one of three things can happen: the light can be absorbed and the energy converted to heat, as when the sun warms something; it can pass through the object, as when the sun's rays hit water or glass; or it can be reflected, as in the case of a mirror or any light-colored object, such as a piece of chalk. Often two or all three of these happen; for
   
 



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