458 



THE SENSORIAL FUNCTIONS. 



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 op- 

 tical instrument called the Camera Ohscura, 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 exaggerated : for on ordinary occasions, where great nicety is 



411 



not required, this difference in the degree of convergence be- 

 tween the central rays and those near the circumference of the 

 lens, giving rise to what is termed the Aberration of Sphericity , 

 is too small to attract notice. 



