VISUAL SENSATIONS, FROM EXCITATION OF THE RETINA 653 



nerve into the retina, where the rods and cones are absent, is insensitive to 

 light and is called the blind spot. The phenomenon itself is very readily 

 demonstrated. If we close one eye, and direct the other upon a point at 

 such a distance to the side of any object that the image of the latter must 

 fall upon the retina at the point of entrance of the optic nerve, its image is 

 lost. If, for example, we close the left eye, and direct the axis of the right 

 eye steadily toward the circular spot in figure 467, while the page is held at 

 a distance of about six inches from the eye, both dot and cross are visible. 

 On gradually increasing the distance between the eye and the object, by 

 removing the book farther and farther from the face, keeping the right eye 

 steadily on the dot, it will be found that suddenly the cross disappears from 

 view, while on removing the book still farther it suddenly comes into view 

 again. The cause of this phenomenon is simply that the portion of retina 

 which is occupied by the entrance of the optic nerve is quite blind; and there- 

 fore that when it alone occupies the field of vision objects cease to be visible. 



-I- 



FIG. 467. Diagram for Demonstrating the Blind Spot- 



2. In the fovea centralis and macula lutea, which contain rods and cones but 

 no optic-nerve fibers, light produces the greatest effect. In the latter, cones 

 occur in large numbers, and in the former cones without rods are found, 

 whereas in the rest of the retina, which is not so sensitive to light, there are 

 fewer cones than rods. We may conclude, therefore, that cones are even 

 more important to vision than rods. 3. If a small lighted candle be moved 

 to and fro at the side of and close to one eye in a dark room while the eyes 

 look steadily forward into the darkness, a remarkable branching figure, 

 Purkinje's figures, is seen floating before the eye, consisting of dark lines on 

 a reddish ground. As the candle moves, the figure moves in the opposite 

 direction, and from its whole appearance there can be no doubt that it is a 

 reversed picture of the retinal vessels projected before the eye. The two 

 large branching arteries passing up and down from the optic disc are clearly 

 visible, together with their minutest branches. A little to one side of the disc, 

 in a part free from vessels, is seen the yellow spot in the form of a slight de- 

 pression. This remarkable appearance is due to shadows of the retinal 

 vessels cast by the candle. The branches of these vessels are chiefly dis- 

 tributed in the nerve fibers and ganglionic layers; and since the light of the 

 candle falls on the retinal vessels from in front, the shadow is cast behind 

 them, and hence those elements of the retina which perceive the shadows 

 must also lie behind the vessels. Here, then, we have a clear proof that the 

 light-perceiving elements of the retina are not the fibers of the optic nerve 

 forming the innermost layer of the retina, but the external layers of the retina, 

 the rods and cones. 



