724 HANDBOOK OF PHYSIOLOGY. 



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 appear- 

 ance is 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, rods 

 and cones, which indeed appear to be the special terminations of the 

 optic nerve-fibres. 



Dtiration of Visual Sensations. The duration of the sensation pro- 

 duced by a luminous impression on the retina is always greater than that 

 of the impression which produces it. However brief the luminous impres- 

 sion, 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 apparently 

 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 

 revealed 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 it. 

 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 con- 

 siderable 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 

 intensity of the sensation is not directly proportional to the intensity 

 of the luminosity of the object. It is necessary for light to have a cer- 

 tain intensity before it can excite the retina, but it is impossible to fix ,-m 

 arbitrary limit to the power of excitability. As in other sensations, so 



