66o THE SENSE OF SIGHT. 



the sense of vision, since they afford constantly fresh 

 subject for the action of the mind. 



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 (the duration of 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 or spectrum, produced 

 by an object, is greater in a direct ratio with the dura- 

 tion 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 considerable period, if we have previously kept our 

 eye fixed for some time on it. 



The colour of the spectrum varies with that of the object 

 which produced it. The spectra left by the images of 

 white or luminous objects, are ordinarily white or lumi- 

 nous; those left by dark objects are dark. Sometimes, 

 however, the relation of the light and dark parts in the 

 image may, under certain circumstances, be reversed in 

 the spectrum ; what was bright may be dark, and what 



