$ On the UNEQUAL . 



ployed, than in thofe compofed only of two ; and alfo the ad- 

 vantages in this refpect, which may be derived from introdu- 

 cing fluid mediums, which differ from glafs in their mean re- 

 fractive denfity, and in the quantity of aberration produced by 

 their refractions. For it will be found upon computation, that 

 when the fluid medium is rarer than glafs, the aberration from 

 the fpherical figure is increafed, and becomes greater in pro- 

 portion as its denfity diminifhes. Now, by making the denfity 

 of the fluid medium approach nearer and nearer to the denfity 

 of the glafs with which it is in contact, we may increafe the 

 rarity of our refracting medium, or, which amounts precifely 

 to the fame thing, diminifh the difference of denfity of the 

 two mediums at pleafure. 



It will appear from what has been explained, that the aber- 

 ration from the figure cannot be corrected by interpofing a dif- 

 perfive fluid between two convex lenfes, of a greater refractive 

 denfity than the interpofed fluid. For all the refractions being 

 made the fame way, tend to converge the external rays to 

 points nearer the lens than its geometrical focus. Hence, when 

 rare fluids are made ufe of to remove the aberration from the 

 difference of refrangibility, fome farther contrivance becomes 

 neceffary to correct the fpherical aberration. 



The mofl obvious way, and which on trial I found fuccefs- 

 ful, is to include the rare difperfive fluid between two glaffes, 

 ground concave on one fide and convex on the other, and thus 

 form fuch a concave as lhall be required. By combining this 

 with a convex, an achromatic object-glafs may be formed, as 

 reprefented in the fixteenth figure. The objection to this con- 

 flruction is, that one of the advantages arifing from the ufe of 

 fluids is given up, namely, the prevention of that lofs of light 

 by reflection, which is a confequence of the fluid being in 

 immediate contact with the glafs, whereas in the prefent cafe, 

 the fpace between the convex and concave is occupied by air. 



On 



