158 A FLYS EYE. 



exceedingly difficult process, and it is only lately, after many trials 

 and failures, that I have succeeded in getting sections which make 

 some approach towards perfection. 



In lowly organised animals there are often what are called 

 " eye-spots." These are sensitive to light, but the animals cannot 

 see with them, because there is no lens above them to form an 

 optical image. All eyes that see have a lens to form an image 

 and a substance sensitive to light placed in its focus, like the lens 

 and sensitive plate of a photographic camera. This is a thing 

 that " every schoolboy knows." What is not so well known is 

 that the lens is generally a doublet ; i.e., two lenses, one behind 

 the other. The cornea, usually only thought of as a protective 

 covering, is in the higher animals just as much a lens as the 



teuT^a 



"t^";Sv^ 



Single slice or section of a lly's head, showing,' brain, eyes, etc., 

 magnified 20 diameters. 



double convex crystalline lens behind it. The function of this 

 crystalline lens is to accommodate the focus of the combined 

 lenses to the distance of the object looked at, by the power the 

 animal possesses of altering the convexity of the surfaces of the 

 lens through muscular action. One would hardly expect to find 

 such complicated mechanism in insects ; yet their eyes too have 

 '' doublet lenses," although the hinder one has no power of altering 

 its focus. In insects the cornea is obviously a lens— or perhaps I 

 should say " lenses," since there are thousands of them in some 

 species. But the crystalline lens is so modified that in a blow- 



