Sir D. Brewster on the Action of Light upon the Retina. 169 



Mr. Smith, in the true spirit of philosophy, invites, and I hope 

 I shall do it with the impartiality which truth requires. 



In performing the fundamental experiment in candle-light, 

 I find that the colour of the paper seen by the excited eye 

 varies with the distance of the image from the excited part of 

 the retina. When the image of the paper is at the furthest 

 possible distance from the luminous or exciting image, the 

 colour is yellowish, becoming greenish yellow, green, blue, dirty 

 purple, and finally disappearing as the image approaches to 

 the point of maximum excitation. — The cause of these changes 

 is obvious : the part of the retina least excited, because furthest 

 removed from the exciting cause, becomes insensible to the 

 red rays ; nearer the point of excitation it becomes insensible 

 to the orange also, nearer still to the yellow, nearer still to the 

 green, and so on, till close to that point it is insensible to all 

 light whatever. Hence we have the paper first yellowish, or 

 a mixture of all the rays except red; next greenish yellow, or 

 a mixture of all the rays except red and orange; next green, 

 and so on with the other colours. 



Let us now attend to the red image in the unexcited eye. 

 This red image does not change its colour, while the other 

 image is passing from yellow up to dark purple; which it ought 

 to do if the colours were complementary ; nor does the red 

 change its intensity as it ought to do if the sensibility of the 

 one eye to red light was increased in the same proportion as 

 the sensibility of the other eye to that light is diminished. 



But even the red colour of the unexcited eye is very unde- 

 cided. If when we see it brightest we quickly shut the ex<- 

 cited eye, its redness becomes instantly much less decided, and 

 in like manner if we shut the unexcited eye, the greenness of 

 the other image is much less brilliant. This diminution of 

 tint does not arise in the first case from the eye being shut to 

 the exciting light, for the colours do not disappear with such 

 rapidity ; and in the second case the exciting light still acts. 

 The true cause of the diminution of tint in both cases is the 

 want of contrast, in virtue of which the green image becomes 

 greener in the presence of the red one, and the red image 

 redder in the presence of the green one. 



In the paper already referred to (see this Journal, vol. i. 

 p. 172.), I have stated that red light predominates in candle- 

 light ; and I have found that in a spectrum from a candle the 

 intensity of the blue rays is much less, and that of the red 

 rays much greater, than in the solar spectrum. Hence as this 

 red tinge is increased by contrast with the green image, we 

 obtain a more simple explanation of the apparent affection of 

 the left eye, than by resorting to an action of the brain. 



Third Series. Vof. 2. No. 9. March 1833. Z 



