CHAPTER 



3 



Physical Aspects 

 of Vision 



Some of the physical aspects of vision are accessible to those who have 

 some knowledge of statistical analysis. There are many kinds of studies 

 that have been made, and we shall deal with several of these. 



SENSITIVITY OF THE EYE 



One important question about vision has to do with the number of 

 photons required for seeing. The question has to be refined, because an 

 immediate problem arises in deciding where these photons must be ab- 

 sorbed. Do they all have to be absorbed in the same rod (cones are 

 neglected, since we know they are used for high light-intensity vision) ? 

 Can a single rod effectively absorb more than one photon in any arbi- 

 trarily short time interval? Do rods or groups of rods cooperate in vision? 

 Do they cooperate in experiments to determine the minimum number of 

 photons that may be detected? 



Studies of the sensitivity of the eye to light are carried out by ex- 

 posing the eye to flashes of dim light and measuring the fraction of times 

 (the probability) that the flash is seen. If light of any intensity could 

 be seen, the experiment would be uninteresting. But the results of these 

 experiments are more than normally interesting. 



There are two ways of doing the experiment. The most direct way is 

 to shine progressively dimmer light flashes into the eye and to determine 

 the smallest intensity that is normally seen. The actual mechanics of 

 this experiment is of interest. The subject sits in a dark box with his 

 teeth gripping a plaster cast specially made to fit the subject's teeth. 

 This, plus some clamps on the subject's head, permits him to gaze 

 directly ahead at a very dim red light which also helps to fix the subject's 

 eyes. The rods are quite insensitive to red, so this light doesn't disturb 

 the experiment. Dark-adaptation was achieved by having the subject 

 sit in total darkness for 45 minutes. An aperture which can be illumi- 

 nated with light of arbitrary intensity and wavelength is situated at an 

 angle of about 20 degrees away from the line of sight. The size of the 

 aperture is chosen so that the image formed on the retina covers only 



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