1871.] 255 [Jeffries. 



spending to it. Hence naturalists and philosophers seemed forced 

 to accord to this form of visual apparatus a different method of per- 

 ceiving light from that which prevails with the eyes formed on the 

 principle of the camera obscura. Johannes Muller's dictum, more than 

 anything else, seemed to render this an accepted truth. Long ago, 

 however, the strangeness was pointed out, of an animal having eyes 

 near each other, whose methods of receiving and perceiving light were 

 on two entirely different plans. Mr. Darwin saw at once this would 

 militate against his theory, and comparatively recent research shows 

 that it is not true. These compound or facetted eyes are also now 

 found to have a refracting medium, in the plane of whose focus is a re- 

 cipient surface corresponding to a retina. Each one of these facets is 

 in reality a convex lens, and as an old anatomist said, " if we look at 

 a man through these we shall see a whole army of dwarfs." There is, 

 then, a picture formed behind them, just as there is a picture formed 

 on the retina in the vertebrate eye. Moreover, behind each facet 

 there is a refracting bouy which we will call the vitreous cone, and 

 however its shape and appearance may vary in insects and crustace- 

 ans, yet. its purpose remains the same; namely, that of refracting the 

 light, and together with the convex facet focussing it on the terminal 

 end of the optic nerve fibre behind and in contact with the vitreous 

 cone. Here, then, the stimulus of light produces excitation of a 

 nerve to carry sensation to the brain, or its representative. The 

 facet may represent the human cornea, the refracting vitreous 

 cone next behind it, the crystalline lens, and if we should push 

 back the final optic nerve termini by the interposition of a 

 vitreous humor, the very shape would then resemble the ver- 

 tebrate type. Thus, we find unity of design in all eyes, ver- 

 tebrate, simple and compound. The question naturally arises, how 

 can the insect see things singly if thousand of pictures of the 

 same thing are perceived. The answer is that a single fibre supplies 

 many facets. Moreover, eyes seemingly facetted or compound, are 

 on examination, found to be groups of simple eyes close together. 

 No objection has been made to an animal's seeing singly with several 

 simple eyes, when these are closely grouped, or to man's single vision 

 with two eyes. A multiplied picture does not go as such to the 

 brain. 



Now then, where does light become turned into nerve stimulation V 

 This takes place in the retina, for the optic nerve itself is insensible 

 to light, and where it enters the eyeball is a blind spot in our field of 



