458 THE SENSORIAL FUNCTIONS. 



venture to enlarge the aperture through which 

 the Hght was admitted into our dark chamber, 

 and fit into the aperture a double convex lens. 

 We have thus constructed the well kiiown op- 

 tical instrument called the Camera Obscuta, in 

 which the images of external objects are formed 

 upon a white surface of paper, or a semi-trans- 

 parent plate of glass ; and these images must 

 evidently be in an inverted position with re- 

 spect to the actual objects which they re- 

 present. 



Such is precisely the construction of the eye, 

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

 in both these instruments, the objects, the prin- 



distant than that to which the remoter rays converge: an effect 

 which I have endeavoured to illustrate by the diagram Fig. 411 ; 

 where, in' order to render it obvious to the eye, the disparity 

 is much exaggerated. But on ordinary occasions, where great 



411 



nicety is not required, this difference in the degree of convergence 

 between the central rays and those near the circumference of the 

 lens, giving rise to what is termed \\\e Aberration of Sphericity , 

 is too small to attract notice. 



I 



