THE RETINA 



839 



terminations of this kind, it is important to have a uniform and moder- 

 ate intensity of illumination, because visual discrimination markedly 

 increases with the light until a certain upper limit has been reached. 



Purkinje's Figures. — The fact that the sensory elements of the retina 

 are deeply seated, is also proved by the phenomenon commonly known 

 as '^Purkinje's images. "^ It has been pointed out that the blood- 

 vessels of the retina ramify upon its inner surface, whereas the rods 

 and cones constitute its outermost layer. Consequently, it might be 

 supposed that all light entering the eye must cast a shadow of the blood- 

 vessels upon these sensitive elements. Actually, however, a dis- 

 turbance of this kind is obviated by the fact that the diameter of even the 

 largest retinal vessel amounts to only one-sixtK of the thickness of the 

 retina, while the diameter of the pupil equals 

 only about one-fifth of the distance between 

 this orifice and the fundus. Under ordinary 

 conditions, therefore, the rods and cones are 

 the recipients of the penumbra of the blood- 

 vessels, while their umbra falls upon the inner 

 layers of the retina. Experimentally, how- 

 ever, we can make use of two or three differ- 

 ent means to render them visible by throwing 

 their shadows upon parts of the retina not 

 ordinarily exposed by them. 



If the eye is turned inward and is directed 

 upon a dull background while the attendant 

 reflects a beam of light upon the outer sur- 

 face of the sclera directly behind the cornea, 

 an arborescent image of the blood-vessels of 

 the illuminated part of the eyeball will be ob- 

 tained (Fig. 442). In this case, blood-vessel 

 B throws a shadow upon the neighboring re- 

 tina opposite the beam of light AB. If the 



latter is then moved one way or another, the image of these vessels is 

 shifted in the same direction. Naturally, this stimulation at C is 

 projected into space through the optical axis as apparently having 

 come from D. This method may also be employed to calculate the 

 distance between the blood-vessels and the sensory elements of the 

 retina, the factors necessary for this calculation being the distance of 

 the background from the eye, the dimensions of the eyeball, the angle 

 through which the light is moved, and the apparent movement of the 

 image upon the screen. The values obtained in this way vary be- 

 tween 0.17 and 0.36 mm. Since it has been determined by histological 

 measurements that the rods and cones lie at a distance of from 0.2 to 

 0.3 mm. below the blood-vessels, we have every reason to suppose 

 that the rays of light are received by these particular constituents of 

 the retina. 



Fig. 442. — Diagram to 

 Illustrate Purkinje's Fig- 

 ures. 



A, source of light; B, 

 blood-vessel; C, shadow 

 thrown by it, which stimu- 

 lation is projected to D upon 

 the screen. 



1 Beitr. zur Kenntniss des Sehens, Prag, 1819. 



