THE SENSES. 605 



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 further, it suddenly comes insight 

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

 ina which is occupied by the entrance of the optic nerve, is quite blind; 

 and therefore that when it alone occupies the field of vision, objects cease 

 to be visible. (2.) In the fovea centralis and macula lutea which contain 

 rods and cones but no optic nerve-fibres, light produces the greatest 

 effect. In the latter, cones occur in larger 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 con- 

 clude, 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 float- 

 ing 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 to- 

 gether 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 

 depression. This remarkable appearance is doubtless due to shadows of 

 the retinal vessels cast by the candle. The branches of these vessels are 

 chiefly 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 of the retina are not. 

 the fibres of the optic nerve forming the innermost layer of the retina,, 

 but the external layers of the retina, almost certainly the rods and cones, 

 which indeed appear to be the special terminations of the optic nerve- 

 fibres. 



Duration of Visual Sensations. The duration of the sensation 

 produced by a luminous impression on the. retina is always greater than 

 that of the impression which produces it. However brief the luminous 

 impression, the effect on the retina always lasts for about one-eighth of 

 a second. Thus, supposing an object in motion, say a horse, to be re- 



