1 72 Sir D. Brewster on the Undulations excited in the Retitia 



nerves*. This conclusion, however, was the result of an im- 

 perfect examination of the phaenomena ; and I am persuaded, 

 from a number of new experiments, that the following is the 

 true explanation of the colours which characterize the two 

 images. 



When the light of the candle held close to the right eye 

 acts upon one part of the retina, it renders every other part of 

 the retina insensible, in a greater or less degree, to all other 

 luminous impressions. The insensibility is a maximum close 

 to the illuminated spot, and diminishes with the distance from 

 it. Objects moderately illuminated actually disappear in the 

 vicinity of the highly excited portion, and bodies of brilliant 

 colours are not only shorn of their spendour, but have their 

 tints entirely changed. 



Dr. Smith observed that a light red slip of paper was seen 

 of a deep red colour by the excited eye, and nearly 'white by 

 the protected eye ; while a. faint gree?i slip appeared a stronger 

 green to the excited eye, and almost "white to the protected 

 one. If we use a stick of red sealing-wax it becomes of a 

 dai-k liver colour to the excited eye, and of a brilliant red to 

 the other eye. All bright blue colours have their intensity 

 diminished in the excited eye; but those which are mixed 

 with the less refrangible rays, or even with white light, be- 

 come of a darker blue, that is, the depth of colour is increased, 

 though the intensity of illumination is diminished. In the 

 case of a compound red colour, such as that of red heat, the 

 image seen by the excited eye is a decided yellow. 



From these results it is obvious, that when the retina is ex- 

 cited by a strong light, the part of the membrane on which 

 the light does not fall is rendered partially insensible to all 

 colours, but in the greatest degree to red light. Hence it fol- 

 lows, that the white slip of paper should appear of a bluish- 

 green colour, the complementary colour of red light. The 

 red tinge which affects the slip of paper seen by the protected 

 eye is the natural colour of candle-light heightened by the 

 contrast of the green slip. As there is far less red in day-light 

 than in candle-light, the slip seen by the protected eye is very 

 much whiter in the former than in the latter. The sensibility 

 of the illuminated part of the retina is affected in the very op- 

 posite manner. It becomes first insensible to blue light, a 

 fact which is clearly proved by the experiments of ^Elpinus 

 and others. 



The influence of light on parts of the retina upon which it 

 does not fall, is finely exhibited in an experiment which has 



* Sec t\r\. Accidental Colours, Edinburgh Encyclopxclia, vol. i. 



