REFRANGIBILITY of LIGHT. 43 
3 
1 
yond the focus at B. Thefe colours may alfo be feen, when 
an image of any luminous object, as the fun, is formed by a 
-lens upon a white ground; and they will be fo much the more 
confpicuous, by how much the diameter of the lens is greater, 
in proportion to its focal diftance. 
Just the reverfe of this will happen in a compound objea- 
glafs, if, in correcting the colour, the medium employed dif- 
perfes more than it ought to do, A blue fringe will then ap- 
pear round a luminous objeét, when the eye-glafs is pufhed in; 
and ared fringe, when it is drawn out beyond what is neceflary 
for diftin@ vifion. 
In this way, the correction of the colour- may be examined, 
and the qualities of refracting mediums inveftigated, to an ex- 
treme degree of accuracy ; yet the effect will be rendered ftill 
more fenfible, by covering half the obje&-glafs. For when 
this is done, the colour produced by the uncovered half of the 
objet glafs appears, without being mixed with that-of the op- 
pofite fide, even when the eye-glafs is adjufted to diftiné vi- 
fion. Thus, in Fig. 13. the colours produced by both fides of 
the lens, are mixed at the general focus F. But if the rays co- 
ming from one fide be intercepted, thofe which are refraCted 
by the other fide will appear in their proper colours. By thefe 
means, and by employing a very luminous object, furrounded 
by a dark ground, and a high magnifying power, the leaft un- 
corrected colour may be rendered fenfible. is 
My firft obfervations, which clearly proved the correction of 
colour which is obtained by the combination of two mediums 
differing in difperfive power, to be only partial, were made in 
the fummer of the year 1787, at Merchifton. 
I uap, fome time before, found it impoffible to fucceed, in 
this refpe&t, with prifms compofed of crown and flint glafs. 
_ But as I neither was able to make’ the phenomena fo apparent 
by this method as with lenfes, nor had a command of prifms 
with that great variety of refraGting angles neceflary to put it 
wer. Ui.” x. beyond 
