Biographical Memoir ofDr Thomas Young. 221 



on the interesting subject of vision which deserve the attention 

 of the historian. There (without however, Hke the philoso- 

 pher of ^Egina, having haughtily interdicted from his school 

 those who were not geometers), the prudent experimenters 

 pointed out the only road by which man can succeed, without 

 committing errors, in the conquest of unknown regions ; it was 

 there that Maurolycus and Porta proclaimed to their cotem- 

 poraries, that the problem to discover that which is, presents 

 so many difficulties, that it is at least presumptuous for any 

 to throw themselves " into the ivorld of intelligihles,^ and to 

 seek for what ought to be ; it was there that the two celebrated 

 countrymen of Archimedes tried to unveil the part performed 

 by the different media of which the eye is composed, and to 

 shew that they were willing, as were Galileo and Newton af- 

 terwards, not to raise themselves above that knowledge which 

 might be elaborated or controlled by the senses, and which, 

 under the portico of the academy, was stigmatized by the dis- 

 dainful epithet of a simple opinion. Such, however, is human 

 weakness, that, after having followed, with extraordinary suc- 

 cess, the principal inflections of the rays of light across the cor- 

 nea and the crystalline lens, Maurolycus and Porta, when on 

 the point of attaining their end, stopped all of a sudden, as be- 

 fore an insurmountable difficulty, when they found their theory 

 opposed by the opinion that objects should appear to the mind 

 reversed, if the images in the eye were turned upside down. 

 The bold genius of Kepler, however, did not allow him to be so 

 staggered. It was from metaphysical reasoning (psychology) 

 that the attack originated, and it was by reasoning, clear, pre- 

 cise, and mathematical, that the objection was overturned. Un- 

 der the powerful hand of this great man, the eye became pre- 

 cisely the simple optical apparatus, known under the name of 

 the camera ohscura; the retina the surface on which the picture 

 is reflected ; the crystalline humour being in the place of a com- 

 mon lens. 



This comparison, which has been generally adopted since the 

 days of Kepler, gives rise to only one difficulty. The camera oh- 

 scura, as in a common telescope, must be put at its right focus, 

 according to the distance of the objects. When these objects 

 approach, it is necessary to withdraw the reflecting surface from 



