36 On the UNEQUAL 
ployed, than in thofe compofed only of two; and alfo the ad- 
vantages in this refpect, which may be detived 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. 
Ir 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 
neceflary to correct the fpherical aberration. 
Tue moft obvious way, and which on trial I found fuccefs- 
ful, is to include the rare difperfive fluid between two glafles, 
ground concave on one fide and convex on the other, and thus 
form fuch a concave as fhall 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- 
ftruCtion 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 {pace between the convex and concave is occupied by air. 
On 
