XXVI. Observations on the Action of Light upon the Retina; 

 with an Examination of the Phenomena described by Mr. 

 Smith of Fochabers. By Sir David Brewster, LL.D. 

 F.R.S. 



|"N calling the attention of philosophers to this curious and 

 *- important class of phaenomena, I presume that the reader 

 has perused my Observations on Mr. Smith's experiment*, 

 and the ingenious paper which has been since published by 

 that gentlemanf, but which was written before the publication 

 of my paper. 



Mr. Smith has described so minutely the mode of perform- 

 ing his beautiful experiment, and has stated the general phe- 

 nomenon so distinctly, that it may be sufficient to observe 

 that when a candle is held near the right eye so as to be seen 

 by it, but not by the left eye, and when both eyes look at a 

 narrow stripe of white paper so as to see it double, the image 

 of the paper seen by the right, or excited eye, will be green, 

 and that seen by the left, or eye protected from the candle- 

 light, will appear reddish. 



Mr. Smith has concluded from a series of ingenious obser- 

 vations, that the light applied to the right eye actually in- 

 fluences the vision of the left eye in virtue of an action of the 

 brain; that the green and red colours are complementary to 

 oneanother; and that the green colour is owing to a dimi- 

 nished sensibility of the right eye to red light, and the red 

 colour to an equally increased sensibility of the left eye to 

 green light. From these results he has deduced the existence 

 of two " functions hitherto unknown," which are excited in 

 the brain by " indistinctness of vision," and the object of 

 which functions is to " remove more or less the exciting cause, 

 and produce distinct vision." — " I forbear," says Mr. Smith, 

 " from making any observations on the singular nature of the 

 cerebral functions thus detected, or on the perhaps still more 

 singular nature of their exciting causes, thinking it due to truth, 

 in a case that appears to involve principles entirely new, to 

 wait the observations of competent inquirers, with whom it 

 remains to confirm or refute, by an impartial scrutiny, the 

 results which I have obtained." 



Having had occasion to pay considerable attention to this 

 class of phaenomena, and having, in the paper already referred 

 to, arrived at a result opposite to that obtained by Mr. Smith, 

 I feel it incumbent upon me to undertake the scrutiny which 



5m 

 * Sec this Magazine, vol. i. p. 171. t Ibid. pp. 249 and 34S. , 



