CH. LV1II.] FUNCTIONS OF THE RETINA 835 



closed, 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, this image is lost. If, for example, we 

 close the left eye, and look steadily with the right eye at the dot 



here represented, while the page is held about six inches from the 

 eye, both dot and cross are visible. On gradually increasing the 

 distance between the page and the eye, still keeping the right eye 

 steadily on the dot, it will be found that suddenly the cross dis- 

 appears from view, because its image has fallen on the blind spot ; 

 on removing the book still farther, it comes in sight again. The 

 question has arisen why we are not normally conscious of a gap in 

 the image. We can only say that owing to the spot being blind from 

 birth onwards we have come to neglect its blindness, and to interpret 

 our experience as if the blind spot always gave rise to the same 

 visual sensations as are evoked by the neighbouring retinal regions. 



(2) In the fovea centralis which contains the bacillary layer, 

 but in which the other layers of the retina are thinned down to a 

 minimum, light produces the greatest effect. In the macula lutea, 

 cones occur in large numbers, and in the fovea centralis 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. 



(3) If a small lighted candle is moved to and fro at the side of 

 and close to one eye in a darkened room, while the eyes look steadily 

 forward on to a dull background, 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 pro- 

 jected before the eye.* This remarkable appearance is due to 

 shadows of the retinal vessels cast by the candle; and it is only 

 when they are thrown upon the retina in an unusual slanting 

 direction that they are perceived. The branches of these vessels are 

 distributed in the nerve-fibre 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 are 

 not the inner, but one of the external layers of the retina ; further 

 than this, calculation has shown it is the layer of rods and cones. 

 The data for such a calculation are the dimensions of the eyeball, 



* Purkinje's figures can be much more readily seen by simply looking steadily 

 down a microscope, and moving the whole instrument backwards and forwards, o'r 

 from side to side, while so doing. 



