4]0 



THE SENSORIAL FUNCTIONS. 



The refraction by spherical surfaces does not, 

 strictly speaking, 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 con- 

 verge : an effect which I have endeavoured to illus- 

 trate by the diagram Fig. 411; where, in order to 



411 



render it obvious to the eye, the disparity is exagge- 

 rated : for on ordinary occasions, where great 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 the Aberration of Sphericity, is too 

 small to attract notice. 



414 



With a view of simplifying the subject, I have 

 assumed, that the rays which arrive at the eye are 



