216 HAND-BOOK OF PHYSIOLOGY. 



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 de- 

 pression. 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 revealed 

 on a dark night by a flash of lightning. The object would be seen appar- 

 ently for an eighth of a second, but it would not appear in motion; 

 because, although the image remained on the retina for this time, it was 

 really revealed for such an extremely short period (a flash of lightning 

 being almost instantaneous) that no appreciable movement on the part of 

 the object could have taken place in the period during which it was re- 

 vealed to the retina of the observer. And the same fact is proved in a 

 reverse way. The spokes of a rapidly revolving wheel are not seen as 

 distinct objects, because at every point of the field of vision over which 

 the revolving spokes pass, a given impression has not faded before another 

 comes to replace it. Thus every part of the interior of the wheel appears 

 occupied. 



The duration of the after-sensation, produced by an object, is greater 

 in a direct ratio with the duration of the impression which caused ft. 

 Hence the image of a bright object, as of the panes of a window through 

 which the light is shining, may be perceived in the retina for a consider- 

 able period, if we have previously kept our eyes fixed for some time on 

 it. But the image in this case is negative. If, however, after shutting 

 the eyes for some time, we open them and look at an object for an instant, 

 and again close them, the after-image is positive. 



Intensity of Visual Sensations. It is quite evident that the more 

 luminous a body the more intense is the sensation it produces. But the 



