SYSTEMATIC SYNTHESIS OF TISSUES INTO ORGANS. 295 



762. Illus. If TWO LIGHTED CANDLES are placed at a little distance 

 apart, and a lens before them, the light from them, striking on the lens, 

 will be reflected to two foci, and if a hundred lighted candles were sim- 

 ilarly placed there would be a hundred foci. 



763. Remark. A CANDLE, though the best, is NOT A FAIR ILLUSTRA- 

 TION, since its flame is composed of millions of points, from each of 

 which light passes to the lens, through which millions of foci are formed 

 so near together as to cause the points of the flame to appear contiguous ; 

 yet each focus is independent in its cause and all its conditions. 



764. IT IS THEREFORE POSSIBLE FOR THE EYE TO BE 



FURNISHED WITH transparent media, by passing through 

 which the light thrown into the eye from any minute 

 point will be gathered to a corresponding focus. 



765. Remark. THESE POINTS, FROM WHICH LIGHT COMES TO THE EYE, 

 and which are the true objects of vision, are the minutest distinguishable, 

 and will be called visual points, or visual objects. They are to be care- 

 fully distinguished from general objects that, to be sure, may be, single 

 visual points, but are usually composed of myriads of them. A tree, a 

 limb, a twig, a leaf, or a spot upon it, is called, in general terms, an ob- 

 ject. Again, visual points, examined under the microscope, are found to 

 be composed of still smaller points, that can thus be distinguished. 

 Still again, some eyes appreciate points smaller than those seen through 

 other eyes, for some can see blue and yellow where others see green : 

 some painters will be found to declare those colors well mixed, when 

 others will see them streaked, and require that they be still more inti- 

 mately mixed before they will allow that a good green has been pro- 

 duced. We are now prepared to describe 



The Media of the Eye. 



766. THE MEDIA OF THE EYE are four : the Cornea, 

 the Aqueous, Humor, the Lens, and the Vitreous Hu- 

 mor. 



767. THE FR,ONT OF THE EYE is CLOSED by a trans- 

 parent window, 2, called the cornea (horny part, though 

 not composed of horny matter). It occupies about one 

 fifth of the sphere, is more convex than the sclerotic, or 

 is like a section of a smaller sphere inserted in a larger 



762. What said of ? T63. Why ? 764. What is ? 765. What said of ? 

 766. What said of ? 767. How is -? 



