SECT, xx.] ACTION OF LIGHT ON THE RETINA. 199 



eighth part of a second, after the exciting cause has ceased. 

 Every one must have observed, when a strong impression is 

 made by a bright light, that an object remains visible for a 

 short time after shutting the eyes, which is supposed to be 

 in consequence of the continued vibrations of the fibres of 

 the retina. Occasionally the retina becomes insensible to 

 feebly illuminated objects when continuously presented. If 

 the eye be turned aside for a moment, the object becomes 

 again visible. It is probably on this account that the owl 

 makes so peculiar a motion with its head when looking at 

 objects in the twilight. It is quite possible that many 

 vibrations may be excited in the ethereal medium incapable 

 of producing undulations in the fibres of the human retina, 

 which yet have a powerful effect on those of other animals 

 or of insects. Such may receive luminous impressions of 

 which we are totally unconscious, and at the same time 

 they may be insensible to the light and colours which affect 

 our eyes, their perceptions beginning where ours end. 



