44 



PHYSICAL ASPECTS OF VISION 



is not necessary to measure the absolute intensities, so long as we know 

 the ratio of the intensities. 



It is of interest to point out that this experiment could have been done 

 in precisely this way 150 or more years ago. If someone had had the 

 inspiration to fit the results with the already known Poisson Distribution, 

 the quantum nature of light could have been discovered that long ago. 



STRUCTURE OF THE RETINA 



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 Miiller 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 Fig. 20(a). An object such as the 

 arrow to the left will send light into many of the rods shown schemati- 

 cally 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. 



(a) 



(b) 



(c) 



Fig. 20. A schematic representation of the problem of detecting the shape of 

 an arrow as seen by retinas which are, respectively, Mat, concave, and convex 

 to the arrow. 



Up until the time of Miiller 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 



