CHAPTER IX. 



OF THE SENSE OF SIGHT. 



ALREADY we have remarked how in going from touch to 

 taste, from taste to smell, from smell to hearing, the excitants 

 of the special sensations, the modes according to which the 

 exterior medium impresses the organs of the senses, always go on 

 attenuating themselves. At the outset it is the shock, the coarse 

 contact : then it is the sapid particle ; afterwards comes the 

 odoriferant effluvium ; finally, the simple vibration, the sonorous 

 nerve. From hearing to sight the series continues. In effect, 

 to excite the optical nerve, an undulation of gaseous liquid or 

 solid molecules is no longer needful. The normal excitant of the 

 sense of sight is the most impalpable of all : it is the vibration 

 of the ether, a body, doubtless, not penetrable, as was long 

 believed, for everything which is material is extended and im- 

 penetrable, but at least a gas so rarefied and light, that for 

 our rough clumsy instruments ot physics it has no appreciable 

 weight. 



The eye, whatever may be the type of its structure, may be 

 considered, when it is furnished with its essential parts, as a 

 transparent and refractive apparatus, suitable for concentrating 

 the luminous rays on the expansions of the optical nerve. But 

 the eye is far from being always" complete, and it is very curious 

 to see it perfecting itself little by little as we ascend the animal 

 series. How many pages overflowing with a bombastic admira- 

 tion have been written to vaunt the structure pretendedly 



