REFRANGIBILITY of LIGHT. 15 
A variety of other fluids were examined in the fame way ; 
but not having yet colleéted them into a table, I have only men- 
tioned, in general terms, the moft remarkable. 
Havine been thus fuccefsful beyond my hopes, in difcover- 
ing fluids capable of removing the great imperfection of tele- 
{copes, arifing from the different refrangibility of light, the 
next objeé was, to feleét from this variety thofe which feemed 
beft adapted to optical purpofes. 
Tuere can be no doubt that thofe aaa an which moft 
difperfe the rays, are, ceteris paribus, to be preferred. It will 
alfo be found, when the method of correcting thofe errors, 
which arife from the fpherical figures of lenfes, comes to be 
confidered, that there is apparently an advantage in ufing a dif- 
perfive medium, whofe mean refractive denfity exceeds the mean 
refractive denfity of crown-glafs. 
As the antimonial cauftic poffeffes both thefe advantages, in a 
degree far beyond what was to be expected in any fluid, I in- 
cluded fome of it between two double convex lenfes of crown- 
glafs, whofe radii of convexity. were as two to one. The leaft 
convex fides of thefe were turned towards each other, and they 
were kept at a proper diftance by means of a glafs-ring. The : 
cavity was then filled with the ftrongeft butter of antimony. 
Here it is evident that there is a concave lens of the difperfive 
fluid, aGting in oppofition to the two convex lenfes of crown- 
glafs, and that the proportion of the radii of thefe is the fame 
which was found by the prifms to corre&t the colour, namely, 
three wedges of crown-glafs, to one of the butter of anti- 
mony. 
Tuis compound object-glafs’ being put into a tube, an eye- 
glafs was applied, and, according to expectation, the colour 
was found to be removed. But I was furprifed to find, on di- 
recting the inftrument to a planet, and ufing a deep eyé-glafs, 
that this fluid, in its highly concentrated ftate, was fubject, like 
flint- 
