REFRANGIBILITY of LIGHT. 29 
For it is an axiom in optics, that if a ray of light after re- 
fraction be returned dire@tly back to the point of incidence, it 
will be refracted in the line which was before defcribed by the 
incident ray. 
Ir 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 direftly back from thefe 
points, it will be accurately converged to the point S,- whence 
it iffued. Now, the parallel rays SH, RK, (Fig. 11.) after 
their emergence from the concave lens, in the lines HX, KV, 
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 cnnvarees to the point S, without any aber- 
ration. 
THis, eine is the “ 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 pafling from the external part 
of the concave into the external part of the convex, in immedi- 
ate contact with it; and if the furfaces of the two lenfes, which 
refpeét 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 fenfible i in large apertures. 
Hence may be underftood the reafon why the indiftinétnefs 
arifing from the fpherical figures of lenfes, may, in the com- : 
mon achromatic telefcope, be more nearly removed in thofe 
conftructions of object-glaffes in which three lenfes are em- 
ployed, 
