790 THE EYE AND VISION [oil. LVI. 



the stimulus of light, and transforming it into a nervous impulse 

 which passes to the brain by the optic nerve. 



The bacillary layer, or layer of rods and cones, is at the back 

 of all the other retinal layers, which the light has to penetrate 

 before it can affect this layer. The proofs of the statement that it is 

 the layer of the retina which is capable of stimulation by light are 

 the following : 



(1) The point of entrance of the optic nerve into the retina, 

 where the rods and cones are absent, is insensitive to light, and is 

 called the blind spot. This is readily demonstrated by what is known 

 as Mariotte's experiment. If we direct one eye, the other being 

 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. The gap is not felt for the reason that a defect of light 

 sensations at a spot blind from the beginning can no more be per- 

 ceived as a gap in the image than the blindness, say, of the skin of 

 the back or foot can be so perceived. 



(2) In the fovea centralis which contains 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 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- 



