FUNCTIONS OF THE RETINAL STRUCTURES. 1099 



Hering points out, binocular contrast gives a very delicate means of 

 determining the condition of excitability, i.e. of adaptation of the two eyes. 



Ebbinghaus 1 has described an after-image seen by one eye after 

 stimulation of the other eye, the after-image being of the same colour as the 

 original ; and the same phenomenon, as already mentioned, may be seen 

 in connection with binocular contrast. This kind of after-image must be 

 distinguished from the negative after-image which is ascribed to one eye 

 after stimulation of the other, this phenomenon depending simply on the 

 difficulty of telling with which eye an observation isbeingmade(seep. 1136). 



The subject of binocular colour mixture will be discussed later. 



In order to explain the phenomena of binocular contrast, it is necessary 

 either to assume a sympathetic relation between the two retinae or to 

 ascribe them to cerebral processes. The movements, described by 

 Engelmann, of the cones and pigment in one eye, when light is applied to 

 the other eye, give a possible physiological basis for the retinal origin of 

 the phenomena ; but these movements probably take place too slowly, even 

 in warm-blooded animals, to explain the almost instantaneous occurrence 

 of binocular contrast, On the other hand, Engelmann and v. Grijns 

 found that the electrical variation of the opposite eye occurred after a 

 short latent period (p. 1052), and a retinal process underlying this may be 

 the basis of the Tjhenomenon. The alternative is that binocular contrast 

 depends on a cerebral process, and since there seems little reason to 

 differentiate the binocular from the ordinary form of colour contrast, this 

 alternative would render probable the central origin of contrast generally. 



Functions of the Retinal Stkuctuees. 



The sensitive layer. — The facts of light and colour-vision having 

 been reviewed, their relation to retinal structures and processes may 

 now be considered. The chief theories of vision are based on the hypo- 

 thetical existence of certain visual substances, and do not attempt to 

 identify these substances with any known retinal 

 structures. Quite apart from these theories, however, 

 there are some grounds for assigning certain visual 

 functions to certain retinal structures. The reasons 

 which establish the rod and cone layer as the 

 primarily sensitive part of the retina may be dealt 

 with first. 



The changes which have been described in the 

 rods, cones, and pigment epithelium may be taken as 

 evidence that these structures are the sensitive 

 elements of the retina, and that the chief processes 

 by which light vibrations are transformed into nervous 

 impulses take place in them. Further evidence is 

 derived from the entoptic appearances known as 

 Purkinje's figures. These are due to shadows thrown 

 by the retinal vessels on the sensitive elements 

 behind, and are under ordinary circumstances in- 

 visible, owing to local adaptation. The distance of the 

 elements behind the vessels may be calculated from the apparent move- 

 ment of the figures on moving the source of illumination. Purkinje 

 described three methods of seeing the figures : by concentrating light with 



1 Arch./, d. ges. Physiol., Bonn, 1890, Bd. xlvi. S. 498. 



