386 OPTICAL ACTION OF THE EYE. 



Analysis of the vitreous humor shows that it consists of water contain- 

 ing about one and a third per cent, of common salt, with a trace of al- 

 bumen. 



The crystalline lens is a double convex of unequal curvatures, the an- 

 The crystalline terior surface being the flattest, the shape changing with the 

 lens. period of life, as also does the density of its parts, its cen- 



tral portions being always the most dense. In construction it is ex- 

 tremely complex, being made up of fibres ranged side by side, and so 

 forming successive lamina. The fibres are about the -^fa-^ part of an 

 inch thick. The refracting power of the lens differs at the centre and 

 circumference : in the former region it is greater. In chemical composi- 

 tion, the lens consists of about fifty-eight per cent, of water, and thirty- 

 six per cent, of a form of albumen known as globulin. 



The aqueous humor fills up the space between the lens and the cornea : 

 Aqueous humor. ** * s c m posed of water containing about one per cent, of 

 common salt. 



Of the Optical Action of the Eye. 



It is the province of the works on natural philosophy to explain how, 

 Formation of wnen ravs ^ % nt &H upon a convex lens, or upon combi- 

 images by nations of such lenses, an image of the object will form at the 

 proper focal distance. For the purposes of physiology, it is 

 sufficient to receive this as a fact, which may be easily illustrated by ob- 

 serving the images of external objects depicted upon a sheet of white 

 paper when a convex lens or magnifying-glass is held at a particular 

 distance between the object and the paper. 



In making such an experiment, some other facts which concern the 

 Effect of dis- physiologist may be readily demonstrated: 1st. That the 

 tanceoftheob- focal distance, that is, the distance between the lens and the 



ject and curv- . .,,.. /> - 



ature of the paper, is variable : it is greater for objects that are near, less 

 for those that are remote ; 2d. That lenses of different curv- 

 atures being compared together, the flatter ones have the longest focus 

 for objects at the same distance ; 3d. That lenses of the same focus, but 

 of different diameters, give images unequally sharp, an indefiniteness be- 

 ing perceived in the image given by the lens of large diameter. This in- 

 distinctness is due to the spherical figure of the lens, and would not have 

 Spherical and occurred had the surface been ground to another conic sec- 

 chromatic aber- tion. It is called spherical aberration ; 4th. Unless the 

 lens be of very long focus, or its aperture or diameter be 

 very small, the edges of the images it yields will be fringed with rainbow 

 colors, and thereby a second cause of indistinctness arises. It is called 

 chromatic aberration. This aberration may be destroyed by properly 

 combining together lenses made of different refracting media, and with 



