REFRANGIBILlTr of LIGHT. 29 



For it is an axiom in optics, that if a ray of light after re- 

 fraction be returned directly back to the point of incidence, it 

 will be refracted in the line which was before defcribed by the 

 incident ray. 



If therefore we conceive the whole of the light emitted from 

 the point S (Fig. 10.), and converged by the convex lens towards 

 the points D and F, to be returned directly back from thefe 

 points, it will be accurately converged to the point S, whence 

 it ifTued. Now, the parallel rays S H, R K, (Fig. 11.) after 

 their emergence from the concave lens, in the lines H X, K V, 

 are precifely in the fame relative fituation, as the rays fuppofed 

 to be returned directly back from F and D are in, at their in- 

 cidence on the convex ; and therefore, when thefe lenfes are 

 placed contiguous, in the manner reprefented in the twelfth 

 figure, parallel rays incident on the concave lens, and immedi- 

 ately after their emergence from it, entering the convex lens, 

 will be accurately converged to the point S, without any aber- 

 ration, 



This, which is the moil fimple cafe, will fuffice to explain 

 the nature of that aberration, which arifes from the fpherical 

 figures of lenfes, and a method of obviating it by combining 

 a convex and concave. 



The demonftration is perfect as far as regards the external 

 ray, which is here reprefented paffing from the external part 

 of the concave into the external part of the convex, in immedi- 

 ate contact with it j and if the furfaces of the two lenfes, which 

 refpect each other, were either in contact or parallel, it would 

 be true with regard to all the rays. But as this is not the cafe, 

 there arifes a fmall fecondary aberration, the effect of which 

 only becomes fenuble in large apertures. 



Hence may be underftood the reafon why the indiftinctnefs 

 ariung from the fpherical figures of lenfes, may, in the com- 

 mon achromatic telefcope, be more nearly removed in thofe 

 conductions of object-glafles in which three lenfes are em- 

 ployed, 



