262 On the Optical and Physiological Discoveries 



at the method which he chose to repel them. As a fellow of 

 the Royal Society of Edinburgh, he announced his intention 

 of communicating to that body a paper " on the motions of 

 the eye-ball ;" and in order that both sides of the question 

 might be before the society at the same time, Dr Brewster 

 announced a notice, entitled, Further Observations on the Vi- 

 sion of Impressions on the Retina, which we shall give as it 

 was read. 



Farther Observations on the Vision of Impressions on the 



Retina. 



" Having learned from Professor Bell, that the paper on the 

 Motions of the Eyeball, by Mr C. Bell, was an answer to the 

 Observations on the Vision of Impressions on the Retina, 

 which I had the honour of submitting to the Society, and 

 which have for some time been before the public, I take it for 

 granted, that that paper has not carried conviction to Mr BelPs 

 mind. 



As the part of the subject which came under my notice is 

 strictly physical, and is susceptible of experimental proof, and 

 of rigid demonstration, it will not admit of that special plead- 

 ing, by which, in the less accurate sciences, the gravest errors 

 have often gained a temporary ascendancy. The points at 

 issue between Mr Bell and me are too simple in themselves, 

 and too much insulated from physiological mysteries, to ad- 

 mit of being involved, either in the ambiguities of language 

 or of argument ; and if any such cloak is thrown over them, 

 it is easy to strip them of the unnecessary drapery. 



Mr Bell maintains, that when the eye, with a spectral im- 

 pression, is pushed aside by the finger, the spectrum remains 

 absolutely fixed and immoveable. In direct opposition to this, 

 I maintain, that the spectrum does move, because I have seen 

 it move, and have measured the extent of its motion. 



Mr Bell next maintains, that the immobility of the spectrum 

 is a physiological fact, arising from the non-exertion of the pro- 

 per, muscles; whereas I maintain, that the motion which does 

 take place is an optical fact, and that the amount of it actually 

 observed, can be computed from optical principles. 



Mr Bell likewise maintains, that when the eye is closed } the 



