CH. LV.J THE BLIND SPOT. 769 



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 direct the axis of the right 



eye steadily towards the circular spot here represented, 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, and still 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 

 in sight again. The cause of this phenomenon is simply that the 

 portion of the retina which is occupied by the entrance of the optic 

 nerve is quite blind ; and therefore when the images of objects 

 fall on it they cease to be visible. By a psychical process the 

 blind spot is not normally perceived. 



(2) In the fovea centralis and macula lutea which contain rods 

 and cones but no optic nerve-fibres, and in which the other layers 

 of the retina are thinned down to a minimum, light produces 

 the greatest effect. In the latter situation, 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 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 (Purkirtje'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.* 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 



* Purkinje's figures can be much more readily Re<-n l>y simply looking 

 steadily down a inifiM-i'upr. ;m<l muviii.L' tin- \vliulr instrument backwards 

 and forwards, or from side to side, while HO doing. 



K.P. 3 D 



