OX THE RELATION OF OPTICS TO PAINTING. 115 



You must all have observed the dark spots which 

 move about in the field of vision, when we have, been 

 looking for only a short time towards the setting sun, 

 and which physiologists call negative after-images of 

 the sun. They are due to the fact that only those parts 

 of the retina which are actually struck by the image of 

 the sun in the eye, have become insensitive to a new 

 impression of light. If, with an eye which is thus 

 locally tired, we look towards a uniformly bright sur- 

 face, such as the sky, the tired parts of the retina are 

 more feebly and more darkly affected than the other 

 portions, so that the observer thinks he sees dark spots 

 in the sky, which move about with his sight. We 

 have then in juxtaposition, in the bright parts of 

 the sky, the impression which these make upon the 

 untired parts of the retina, and in the dark spots 

 their action on the tired portions. Objects, bright 

 like the sun, produce negative after-images in the 

 most striking manner; but with a little attention they 

 may be seen even after much more moderate impres- 

 sions of light. A longer time is required in order to de- 

 velop such an impression, so that it may be distinctly 

 recognised, and a definite point of the bright object 

 must be fixed, without moving the eye, so that its image 

 may be distinctly formed on the retina, and only a 

 limited portion of the retina be excited and tired, 

 just as in producing sharp photographic portraits, the 



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