PROPERTIES OP LENSES. 13 



to correct the other without decreasing the magnify- 

 ing power to any great extent, The lens A Fig. 10 

 completely destroys the spherical aberration of the 

 lens B, for it does not change the position of the 

 rays situated near its centre as much as it does 

 those at the edges, owing to its increasing prismatic 

 form. 



If advantage be taken at the same time of the 

 different relations between the refractive and dis- 

 persive powers of different kinds of glass, and if the 

 lenses which correct the spherical aberration by their 

 shape are formed from different kinds, chromatic 

 aberration will be destroyed. If the lens A (Fig. 10) 

 of high dispersive power, concave form, and low cur- 

 vature, be joined to B of lower dispersive power and 

 greater curvature, it is evident that the dispersion 

 caused by the latter is neutralised, whilst its refrac- 

 tive power is only decreased by the opposite refrac- 

 tion of the concave. 



The convex lens of crown is generally corrected by 

 a concave of flint, it has been found that no two 

 lenses can be made to correct each other perfectly, 



