VISION. BI9 
and cones is convincing. If it could be shown that parts of 
the retina itself internal to these layers cast perceptible shad- 
ows, the conclusion that the rods and cones are the essential 
parts of the sensory organ would be inevitable. The following 
experiment proves this: When a light is moved backward and 
forward (to prevent retinal fatigue) before the eye, so that the 
rays from it enter the organ, while the subject, standing ina 
dark room, gazes toward a plain-colored wall, his accommoda- 
tion being relaxed, he will behold radiating shadows, somewhat 
suggestive of the leafless branches of an old tree. These cor- 
respond with the picture of the retinal vessels as ascertained 
by an examination of the eye with the ophthalmoscope. Some 
persons always see the shadows of the blood-corpuscles also; 
and, in fact, one physiologist has, by observing these, calculated 
the rate of the blood-flow in the retinal vessels. Instead of 
UO 
Fig. 424, 
Fie. 423. 
Fic. 423.—Ophthalmoscopic picture of fundus oculi, showing the generally red ground (dark 
in Bau), on which may be distinguished the point of entrance of optic nerve, in the 
region of which the prominent vessels may be seen to arise (after Bernstein). 
Fie. 424.—_Diagram to explain experiment to get Purkinje’s figures. In this case the light 
passes through the lens, an image is formed on the retina, and the light is reflected from 
this image to another part of the retina, at which, being less illuminated, the shadows of 
the retinal vessels are more pared perceived. Thus, pose the candle to be held at a, 
its image will be formed at b and reflected to c, at which point shadows appear and are 
projected into the visual field at d.__By moving the candle to a’, we get new relative posi- 
tions for image, vessels, etc. (after Bernstein). 
moving the light to and fro, it may be concentrated for a few 
seconds by a lens with the same result—the appearance of 
Purkinje’s figures, as they are termed. When the light is 
moved, they shift place correspondingly. If the sensory parts 
were not situated behind the retinal vessels, it is impossible to 
conceive how their shadows could be seen, and certain mathe- 
