26 LIFE IN NATURE. 



called sensitive ; by a blind instinct, which is often 

 truer than studied science, for the retina, or expan- 

 sion of the optic nerve within the eye, is like it. The 

 retina consists of matter prone to change. Its ele- 

 ments tend to break tip, and enter into new combi- 

 nations. What supposition can be better warranted 

 than that the rays of light entering the eye permit a 

 change of composition, as they are known to do in 

 respect to the photographic salts ? 



Mr. Grove by a beautiful experiment* has shown 

 that light, falling on a plate prepared for photo- 

 graphy, will set up a galvanic current. Does not 

 this unavoidably suggest itself as an illustration of 

 the process of vision ? Light impinging on the retina 

 determines therein a chemical change, which de- 

 velops in the optic nerve the nervous force. This 

 force sets up in the brain an action of the same order 

 as that in the retina. Hence again originates a 

 nervous force, which, conveyed back to the eye, 

 sets up yet a third time a chemical change (in the 

 iris), which causes the contraction of the pupil. 



* On the Correlation of the Physical Forces. 



