188 



Prof. Helmholtz — Normal Motions of the [April 14, 



coloured strips is projected on the nervous membrane of your eye ; those 

 parts of this membrane on which the Ught falls are irritated, and in con- 

 sequence of this irritation, their irritability is exhausted, they are fatigued 



Fig. 1. 



a 



and they become less sensitive to that kind of light by which they 

 were excited before. When you cease, therefore, to look at the coloured 

 strips, and turn your eye either to the grey ground of the diagram, or to 

 any other part of the field of vision which is of a uniform feeble degree of 

 illumination, you will see a spectrum of the coloured strips, exhibiting the 

 same apparent magnitude but with colours reversed, a narrow green strip 

 being in the middle of a broader red one. The cause of this appearance is, 

 that those parts of your retina which were excited formerly by green light 

 are less affected by the green rays contained in white or whitish light than 

 by rays of the complementary colour, and white light, therefore, appears 

 to them reddish ; to those parts of the nervous membrane, on the other 

 hand, which had been fatigued by red light, white light afterwards appears 

 to be greenish. The nervous membrane of the eye in these cases behaves 

 nearly like the sensitive stratum in a photographic apparatus, which is 

 altered by light during the exposure in such a way that it is impressed 

 differently afterwards by various agents ; and the impression of light on 

 the retina may be, perhaps, of the same essential nature as the impression 

 made upon a photographic plate. But the impression made on the living 

 eye does not last so long as that on sensitive compounds of silver ; it vanishes 

 very soon if the light be not too strong. Light of great intensity, like 

 that of the sun when directly looked at, can develop e very dark ocular 

 spectra, which last a quarter of an hour, or even longer, and disturb the 



