504 NOTES TO BOOK IX. 



structure of the eye. The operation of the parts was 

 gradually made out. Baptista Porta compares the eye to 

 his Camera Obscura (Magia Naturalis, 1579). Scheiner, in 

 his Oculits, published 1652, completed the theory of the eye. 

 And Kepler discussed some of the questions even now often 

 agitated ; as the causes and conditions of our seeing objects 

 single with two eyes, and erect with inverted images. 



(GA.) p 395. After a careful consideration of Sir D. 

 Brewster's asserted analysis of the solar light into three 

 colours by means of absorbing media, I cannot consider 

 that he has established his point as an exception to 

 Newton's doctrine. In the first place, the analysis into 

 three colours appears to me quite arbitrary, granting all 

 his experimental facts. I do not see why, using other 

 media, he might not just as well have obtained other 

 elementary colours. In the next place, this cannot be 

 called an analysis in the same sense as Newton's analysis, 

 except the relation between the two is shown. Is it meant 

 that Newton's experiments prove nothing? Or is Newton's 

 conclusion allowed to be true of light which has not been 

 analyzed by absorption ? And where are we to find such 

 light, since the atmosphere absorbs ? But, I must add, in 

 the third place, that with a very sincere admiration of Sir 

 D. Brewster's skill as an experimenter, I think this expe- 

 riment requires, not only limitation, but confirmation by 

 other experimenters. Mr. Airy repeated the experiments 

 with about thirty different absorbing substances, and could 

 not satisfy himself that in any case they changed the 

 colour of a ray of given refractive power. These experi- 

 ments were described by him at a meeting of the Cam- 

 bridge Philosophical Society. 



(HA.) p. 397. Mr. Chester More Hall, of More Hall 



