48 On the UNEQUAL 



I had now attained the object I was in fearch of, namely, a 

 method of refracting equally all the rays of which light is 

 compofed. Nor was the conftruction of object-glafTes for tele- 

 fcopes, which it afforded, liable to any very material objection. 

 The principal inconvenience arofe from the neceflary depth of 

 the fpheres of the lenfes required, which was now the only re- 

 maining obftacle to fliortening the refracting telefcope at plea- 

 fure. 



In the firfl 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 fucceis, and therefore 

 not at all curious 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. 



The firft fluid I happened to make trial of, was a metallic 

 folution with a mixture of marine acid. Upon comparing an 

 object glafs, rendered achromatic by this folution, with another 

 as nearly fimilar to it as poflible, 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 *. T repeated the experiment frequently, to enable me to 

 judge of the proportion of focal diftance of a compound con- 

 cave, neceffary 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 

 eflential oils. The convex was not only fhorter itfelf, with the 

 fame depth of fpheres, but required a fhallower compound 



concave 



* The 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. 



