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IX. ON CERTAIN ACTIONS OF COLD, WARMTH, AND LIGHT UPON THE 

 CRYSTALLINE LENS. 



Pourfour du Petit has discovered that after the death of a 

 mammal, it is not uncommon to find the crystalline lens opaque. 

 He has found also, that when the lens has become opaque, if we 

 draw it near the flame of a candle or a lamp, it becomes trans- 

 parent again after a few minutes. The same lens, put alter- 

 nately near and far from the flame, may become alternately 

 transparent and opaque. I have endeavored to discover whether 

 it is the light or the warmth of the flame which renders an opaque 

 lens transparent. I have placed between the lens and the flame 

 a layer of mineral salt, which is athermane and transparent. 

 Then the light of the flame could reach the lens, but not its 

 warmth. There has been no action. In another experiment I 

 have exposed to the light passing through the mineral salt a 

 fresh crystalline lens, still perfectly transparent. It became 

 opaque. Now, in a third experiment, I have compared two 

 crystalline lenses, perfectly transparent, one exposed to the 

 action of light passing through the mineral salt, and the 

 other kept in an obscure place. The former became opaque 

 much quicker, and considerably more than the other. There- 

 fore, 1st. It is not light which renders transparent an opaque 

 lens ; 2d. Light does not prevent a transparent lens from be- 

 coming opaque ; 3d. Moreover, light appears to accelerate, if 

 not to produce, the opacification of the crystalline lens. 



As to the influence of warmth, it is certain that it is that 

 which renders transparent an opaque lens : 1st. When left in 

 the atmosphere, at a temperature superior to 70 F. (21 Cs.), a 

 fresh transparent lens remains transparent. 2d. When a lens 

 has become opaque, it is frequently sufficient to keep it exposed 

 to the warmth of the hand, during some minutes, to render it 

 transparent. 3d. The lower the temperature of the atmosphere 

 the quicker transparent lenses become opaque. 



These experiments give a very interesting result, i. e., that 

 light appears able to produce an effect precisely opposite to the 

 effect produced by warmth. 



From these researches I conclude : 



1st. That light, and a low temperature, are favorable condi- 



3* 



