324 



THE SENSORIAL FUNCTIONS. 



a double convex lens; and it has the property 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 admit- 

 ted into our dark chamber, and fit into the aperture a dou- 

 ble 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 sur- 

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

 images must evidently be in an inverted position with re- 

 spect to the actual objects which they represent. 



Such is precisely the construction of the eye, which is, to 



* The refraction by spherical surfaces does not, strictly speaking 1 , unite a 

 pencil of parallel or divergent rays into a mathematical point, or focus; for 

 in reality the rays which are near the central line are made to converge to a 

 point a little more distant than that to which the remoter rays converge: ati 

 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 nicety is not required, this differ- 

 ence in the degree of convergence between 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. 



