7/O THE EYE AND VISION. [CH. LV. 



proof that the light-perceiving elements are not the inner, but 

 one of the external layers of the retina ; further than this, 

 calculation has shown it is the layer of rods and cones. The 

 data for such a calculation are the dimensions of the eyeball, 

 the distance of the screen from the eye, the angle through which 

 the candle is moved, and the displacement of the figure seen. 



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 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 dur- 

 ing which it was revealed to th,e retina of the observer. 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 replaces it. 

 Thus every part of the interior of the wheel appears occupied. 



The after-sensations are called after-images. They are of two 

 kinds, positive and negative. Positive after-images are those 

 which resemble the original image in distribution of light and 

 shade, and colour. Negative after-images which occur after 

 strong, and especially after prolonged, excitation of the retina, 

 are those in which the light parts appear dark, the dark parts 

 light, and the coloured parts of the opposite or contrast 

 colour. 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 considerable 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. 



The Ophthalmoscope. 



Every one is perfectly familiar with the fact, that it is quite 

 impossible to see the fundus or back of another person's eye by 

 simply looking into it. The interior of the eye forms a perfectly 



