248 Memoir of Dr. Thomas Young. 



composed, and shewed themselves satisfied, as Galileo and 

 Newton subsequently were, not to raise themselves above 

 the knowledge susceptible of being controlled by the senses, 

 and which was stigmatized, under the porches of the Aca- 

 demy, by the disdainful qualification of simple opinion. 

 Such is always human weakness, that after having followed 

 with wonderful success, the principal inflexions of light 

 through the cornea and crystalline lens, Maurolycus and 

 Porta, having nearly attained their object, stopped short, 

 suddenly, as before an insurmountable difficulty which 

 opposed their theory, viz., that objects ought to appear up- 

 side down if the images in the eye are themselves reversed. 

 The persevering mind of Kepler, on the contrary, pre- 

 vented him from giving way. The attack is psycological. 

 The objection is overturned by psycological clearness and 

 mathematical precision. Under the powerful influence of 

 this great man, the eye becomes distinctly the simple op- 

 tical apparatus known under the name of obscure chamber, 

 the retina is the picture, the crystalline lens compensates 

 for the vitreous humour. 



This assimilation, so generally adopted after Kepler, gave 

 rise to one difficulty only. The obscure chamber like a com- 

 mon glass must be placed at a focus according to the distance 

 of objects. When the objects approach, it is indispensible 

 to evert the picture by the lens ; a contrary movement is 

 necessary when the objects are distant. To preserve to the 

 images all desirable precision, without changing the posi- 

 tion of the surface which receives them, is then impossible, 

 at least, that the curvature of the lens may always vary. 



Among the different modes of obtaining distinct images 

 nature has afforded a choice ; for man can see with great 

 precision, at very dissimilar distances. The question thus 

 stated, has afforded a subject of great research and dis- 

 cussion to philosophers. Great names figure in the debate. 



Kepler, Descar.es, support the opinion, that the 



whole of the globe of the eye is susceptible of being elon- 

 gated and flattened. 



Porterfield, Zinn, would have the crystalline 



lens to be moveable for the purpose of placing itself at a 

 greater or less distance from the retina. 



Jurin, Musschenbroek, believe in a change in the 



curvature of the cornea. 



