840 THE SENSE OF SIGHT 



The retinal blood-vessels may also be rendered visible by moving a 

 candle to and fro in front of the eye while gazing upon a dark back- 

 ground. In accordance with the foregoing discussion, it will be seen, 

 however, that the candle must be held well to one side of the visual 

 line, otherwise the shadows of the vessels cannot be made to fall 

 upon a lateral zone of the retina which is ordinarily not exposed to 

 the stimulation by these vessels. 



A third method consists in permitting a beam of light to enter 

 through a pin-hole in a cardboard held directly in front of the cornea. 

 In this way, sharply defined shadows of the blood-vessels will be thrown 

 upon the underlying rods and cones, but even now it is necessary to 

 move the cardboard rapidly to and fro in front of the eye, so that the 

 shadows are not allowed to rest upon the same area of the retina for 

 any length of time. 



Chemical and Physical Changes in the Retina on Stimulation 

 by Light. — Having established the fact that the rods and cones are 

 the elements which are most directly concerned with the transforma- 

 tion of the light stimulus into a visual impulse, we are now in a posi- 

 tion to study the manner in which their stimulation is brought about. 

 The theories pertaining to this subject may be classified as follows: 



1. Mechanical imprint theory which holds that the rays of light produce im- 

 pressions upon the retina, similar to those resulting when the tips of the fingers 

 are made to impinge upon a layer of gelatin. 



2. Thermal theory which proposes that the rays of light traversing the retinal 

 elements, generate heat. 



3. Electrical theory which suggests that the waves of light are transformed into 

 electrical energy. 



4. Chemical imprint theory which holds that the rays of light give rise to chem- 

 ical reductions, the retina containing the phototropic substances necessary for the 

 formation of this imprint. 



Though in the present state of our knowledge no absolutely con- 

 vincing proof can be furnished for any one of these conceptions, the 

 chemical imprint theory is by far the most satisfactory, because we 

 are in possession of certain evidence tending to support it. In analogy 

 with the sensitive plate used in photography, it is assumed that the 

 retina contains a phototropic substance which is dissociated by the 

 entering rays of light. The question may then be asked, whether such 

 a substance has actually been isolated. It will be remembered that the 

 outer poles of the rods and cones are situated upon a layer of pigment 

 which has its origin in the adjoining hexagonal cells of the choroid- 

 retinal junction. This pigment possesses a reddish color in amphibia, 

 and a violet color in fish, owls, sheep, and man.^ For this reason 

 it is commonly known as visual purple or rhodopsin. In 1876 BolP 

 made the interesting observation that this formed pigment does not 

 remain stationary, but moves in and out of the aforesaid cellular 



^ H. Miiller, Zeitschr. fiir wissensch. Zoologie, iii, 1851, 234. 

 2 Sitzungsber,, Akad. der Wissensch., Berlin, 1876. 



