VISION. 



409 



with tolerable exactness, to produce the con- 

 vergence of parallel rays to a focus; and by making- 



the denser medium convex on both sides (as shown 

 in Fig. 410), both surfaces will conspire in pro- 

 ducing the desired effects. Such an instrument is 

 termed a double convex lens; and it has the pro- 

 perty of collecting into a focus rays proceeding 

 from distant points. 



Having obtained this instrument, we may now 

 venture to enlarge the aperture through which the 

 light was admitted into our dark chamber, and fit 

 into the aperture a double convex lens. We have 

 thus constructed the well-known optical instrument 

 called the Camera Obscura, in which the images of 

 external objects are formed upon a white surface of 

 paper, or a semi-transparent plate of glass ; and 

 these images must evidently be in an inverted posi- 

 tion with respect to the actual objects which they 

 represent. 



A Such is precisely the construction of the eye, 

 which is, to all intents, a camera obscura : for in 

 both these instruments, the objects, the principles 

 of construction, and the mode of operation are ex- 

 actly the same ; and the only difference is, that 

 the former is an infinitely more perfect instrument 

 than the latter can ever be rendered by the utmost 

 efforts of human art. 



