464 THE POPULAR SCIENCE MONTHLY. 



is replaced by a dim plate of glass. Now cast a glance at Fig. 2 ; and 

 first suppose the box to be round ; next, instead of the wooden wall, 

 an organic tissue ; and the window glazed with a transparent organic 

 coat or tunic, instead of with a crystal lens, and which fulfills the same 

 purpose as a collective lens, and strengthened by one or more lenses 

 placed one behind the other. Farther, suppose, instead of the black- 

 ened inside of the walls of the box, the organic sclerotic coat overlaid 

 from the inside by a second dark-colored tissue ; and, lastly, the retina 

 at the farther end as sensitive plate, and you have certainly an imper- 

 fect, but still, as far as an outline goes, a good general idea of the chief 

 parts of the eye. 



To the clearer understanding of these parts, the figure is provided 

 with letters. 



The tissue marked at the different sections with an S, is the envel- 

 oping tissue called the sclerotic or sclerotic coat. 



To the front, overlapped by the above, lies the transparent tissue, 

 the cornea, C, which represents the window, and at the same time 

 contributes essentially to collecting the rays of light. At the back 

 enters the optic nerve, which, spreading out to right and left within 

 the sclerotic, receives the name retina, likewise marked with an N. 



The chief business of the lens, L, lying well back, and rendered per- 

 fect by the humors (K, aqueous, K', vitreous), which fill the spaces or 

 chambers, is the refraction of the rays, whose admission has already 

 been prepared by the cornea. 



And, lastly, overlying the interior surface of the sclerotic, is the 

 choroid with its pigment, being the substitute for the black paint in 

 the camera. You find it marked with an A. 



Now, if this eye with its cornea, like a camera-obscura with its win- 

 dow, is turned on the objects of the outer world, we shall behold what 

 Fig. 3 shows us : The light proceeding from a point A, beyond the 

 eye, throws a pencil of rays on the cornea ; this is already refracted 

 here and there on the surface of the lens, but in a manner so as to col- 

 lect all its rays again in the one point a of the retina. This a, then, 

 is the image-point of the object-point A. In the same manner, b be- 

 comes the image-point of the object-point B, and all the object- j>oints 

 between A and B will find their image-points on the retina between a 

 and b. In a word, an inverted perspective image of all the objects 

 comprehended in the space A B will be found reflected on the retina. 



Let us now examine a little more closely the structure of the eye, 

 together with the object it is designed to serve, taking the separate 

 parts in the direction from without inward. 



The sclerotic, a stout and not very elastic coat, wants no further 

 description. On the other hand, however, the cornea, as the transpar- 

 ent window, deserves your whole attention. In the construction of 

 the cornea Nature has had to overcome exceptional difficulties. If you 

 remember how apt every organic body exposed to the air is to fall a 



