Visual Education (Jan-Nov 1920)

30 Visual Education will not fall at the same time on the blind spot of the left eye, and when both eyes are used together there is no invisible position for an object within the whole field of vision. There are many analogies between the eye and a camera and, as has been stated, the retina corresponds to the sensitive plate. But in the case of the retina and the sensitive plate the differences are profound. The sensitive plate is simple; the retina is very complex. The photographic plate is coated simply with an emulsion of gelatin and a compound of silver which has the property of undergoing certain chemical changes when it is exposed to light. The lens throws an image of the object at which it is pointed on to the plate, ana the silver compound of the parts thus exposed turns dark upon treatment with suitable chemical reagents. The remainder is washed away before the plate is taken into the open. On the other hand, the retina is a highly complex structure consisting of nine layers of nerve cells, nerve fibers, blood vessels, granules, and rods and cones. When light falls upon the retina chemical, and possibly physical, reactions take place with resulting stimulus of the optic nerve and corresponding impressions on the brain. Only one picture can be obtained on a photographic plate, because when the silver compound has once been darkened the result is permanent. But the effect of light on the retina soon disappears, and one image can succeed another in an almost endless series. In one respect this is an immense advantage, and in another a disadvantage, as compared with the photographic plate. The advantage is obvious. The disadvantage arises from the fact that the stimulus to the optic nerve produced by an image on the retina does not increase with time. If, for example, an object is too faint to be visible in the first few seconds it can not be seen at all. On the contrary, the effects of a faint light on a photographic plate are proportional to the time of exposure. If a few seconds do not give a strong enough image, the exposure may be continued for a few minutes, or even a few hours. In celestial photography such long exposures are often made, and photographs are obtained of objects which are so faint that they are far beyond the reach of the eye even with the aid of the greatest telescopes. The photographic plate distinguishes among colors only by the fact that it is more sensitive to some than to others. The retina, however, is differently affected by different colors. Thomas Young and, later, Helmholtz explained color perception by the theory that the retina contains three kinds of nerve fibers which are sensitive particularly to three kinds of light, namely, violet, green, and red, while being relatively insensitive to the others. This theory, however, does not explain all the facts, and it has been replaced by one due to Hering. According to Hering the retina contains three kinds of substances, each of which is acted on by one kind of light in one way and by another kind of light in the opposite way. Such pairs of colors are complementary, and when they strike the retina at the same time they produce the effect of grey light. These three pairs of oppositely acting colors are white and black, blue and yellow, and green and red. Other colors may be obtained by mixtures of these colors, though they also exist independently. Indeed, as has been stated, there are rays similar to light whose waves are either