206 PHYSIOLOGV. 



a, a, to converge to a point, or focus at d ; and if it was still 

 more convex, they would come to a point still nearer. We 

 are now able fully to understand how it is that the humours, 

 or lenses of the eye collect the light from visible objects, and 

 converge them into an image upon the retina. fThese media, 

 we have seen, are four in number, viz., the cornea, the aqueous 

 humour, the crystalline lens, and the vitreous humour ; each 

 possessing a different density, different sphericity, and con- 

 sequently different refractive powers. 



26. Now let us examine the principle on which an image 

 is formed. This may be shown by holding a common sun- 

 glass opposite a window in a room, and placing a sheet of 

 paper behind it. We immediately see depicted on the paper 

 a perfect image of the window, diminished according to the 

 convexity of the glass, but inverted. The greater the con- 

 convexity, the nearer must the paper be held to the glass to 

 receive the image, and the nearer to the window the glass is 

 held, the larger will the image appear. 



Fig. 13 



In this cut, rays of light pass from the candle, and the 

 hand A, B, and falling on the lens at C, are conveyed to the 

 paper D, which is consequently illumined with the object 

 in the same way as in the above experiment, the window 

 was painted on the paper. In like manner precisely, are 

 images painted on the retina, as is shown in the following 

 cut. 



