48 On the UNEQUAL 
I HAD now attained the objeét I was in fearch of, namely, 
\ 
imethod of refrating equally all the rays of whgch light is 
compofed. Nor was the conftruction of object-glafles for tele- 
fcopes, which it afforded, liable to any very material objection. 
‘The-principal inconvenience arofe from the neceffary depth of 
’ the fpheres of the lenfes required, which was now the only re- 
‘maining obftacle to fhortening the refraQling telefcope at plea- 
fure. . 2 
In the firft trials I made to difcover a difperfive medium 
which fhould feparate the rays in the fame proportion in which _ 
glafs does, I was in hopes of perfect fuccefs, and therefore 
not at all curiots in obferving the breadths of the coloured 
fringes, ftill hoping that the next trial might afford a refraction 
without any colour whatever. I therefore thought it expedient 
to repeat fome of them, with greater attention to that circum- 
ftance. 
Tue firft fluid I happened to make trial of, was a metallic 
folution with a mixture of marine acid. _Upon comparing an 
objeét-glafs, rendered achromatic by this folution, with another 
as nearly fimilar to it as poffible, in which an effential oil was 
employed for that purpofe, the breadth of the coloured fringes 
appeared indifputably much narrower in the former than the 
latter *. I repeated the experiment frequently, to enable me to 
judge of the proportion of focal diftance of a compound con 
cave, neceflary to correct this fecondary colour, upon the prin- 
ciple which hath juft been explained. Upon a comparative 
trial, I found it better to form the compound convex of a com- 
bination of this fluid and glafs, than of a combination of two 
effential oils. ‘The convex was not only fhorter itfelf, with the 
fame depth of fpheres, but required a fhallower compound 
concave 
* Tue caufe, at that time unknown, was, that the folution happened to contain 
an unufual proportion of the marine acid ; as will be underftood from what 
follows. ise ih eh eee 
a 
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a 
etl me fe. te 
roan 
_ ome. 
