One of the problems which perturb biophysicists is the reason for the size and shape of biological structures. In this category comes the question of why we have a concave retina. In the early 1800's, one Johannes Müller was trying to figure out the answer to this question, and in speculating on the problem he remarked on some interesting aspects.
Consider a flat, plane retina, as in (a). An object such as the arrow to the left will send light into many of the rods shown schematically in the retina. As a result, only the presence or absence of an object will be detected by such an eye. If the retina is made concave, as in part (b) of the figure, some of the rods will still see many parts of the arrow, so that little improvement in vision will result. But consider the convex retina in part (c). Here, if the angular opening of each rod is sufficiently small, only a very few rods see the various portions of the arrow, so that what amounts to an erect image of the arrow is formed on this retina.
Up until the time of Müller no such retinal structures were known, and it is to his credit that he took the trouble to go to Nature to discover that this is essentially the insect eye. The rods are not really rods, but are conelike structures called ommatidia, whose angular opening is about one degree. It is therefore not surprising that the resolution of this insect eye is about one degree. This information is gathered by making a grid of black and white bars of such a thickness that at the eye of the insect the individual bars subtend, in various experiments, 0.1 degree, 0.5 degree, 1 degree, 2 degrees, etc. In response to a movement of the 2-degree bars, the insect responds by moving away. When the 0.5-degree bars were used, the insect made no response, thus showing that the fineness of the bars was such that the images overlapped and looked grey to him; he could perceive no motion in these circumstances.
The human eye can be studied in a similar way, and the resolution of 0.01 degree corresponds well with the angle subtended at the lens by neighboring retinal elements. To give an idea of what these numbers mean, a ringer at arm's length subtends about one degree. Thus an insect can just make out the individual fingers of a man one yard away; a human can make them out about 100 yards away.
It should be realized that the experiments described in this chapter were done 20 years ago and that much work of a far more sophisticated nature has been done in the intervening years. Also during that time, enormous success has been achieved in studying the biochemistry of vision. Some biophysicists feel that studies of vision will provide the most direct route to an understanding of the nerve interplay which goes by the name of neurophysiology. Accordingly, the neurophysiology of vision is emerging as one of the most important and most exciting areas of research today.
