116 ON THE RELATION OF OPTICS TO PAINTING. 



object must be stationary during the time of exposure 

 in order that its image may not be displaced on the 

 sensitive plate. The after-image in the eye is, as it 

 were, a photograph on the retina, which becomes 

 visible owing to the altered sensitiveness towards 

 fresh light, but only remains stationary for a short 

 time ; it is longer, the more powerful and durable was 

 the action of light. 



If the object viewed was coloured, for instance 

 red paper, the after-image is of the complementary 

 colour on a grey ground ; in this case of a bluish green. 1 

 Rose-red paper, on the contrary, gives a pure green 

 after-image, green a rose-red, blue a yellow, and 

 yellow a blue. These phenomena show that in the 

 retina partial fatigue is possible for the several 

 colours. According to Thomas Young's hypothesis of 

 the existence of three systems of fibres in the visual 

 nerves, 2 of which one set perceives red whatever the kind 

 of irritation, the second green, and the third violet, 

 with green light, only those fibres of the retina which 

 are sensitive to green are powerfully excited and tired. 



1 In order to see this kind of image as distinctly as possible, it 

 is desirable to avoid all movements of the eye. On a large sheet of 

 dark grey paper a small black cross is drawn, the centre of which is 

 steadily viewed, and a quadrangular sheet of paper of that colour 

 whose after-image is to be observed is slid from the side, so that one 

 of its corners touches the cross. The sheet is allowed to remain for 

 a minute or two, the cross being steadily viewed, and it is then 

 drawn suddenly away, without relaxing the view. In place of the 

 sheet removed the after-image appears then on the dark ground. 



2 See Helmholtz's Popular Lectures, first series, p. 250. 



