REFRANGIBILlTr of LIGHT'. i 5 



A variety of other fluids were examined in the fame way ; 

 but not having yet collected them into a table, I have only men- 

 tioned, in general terms, the moft remarkable. 



Having been thus fuccefsful beyond my hopes, in difcover- 

 ing fluids capable of removing the great imperfection of tele- 

 fcopes, arifing from the different refrangibility of light, the 

 next object was, to felect from this variety thofe which feemed 

 beft adapted to optical purpofes. 



There can be no doubt that thofe mediums 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 pofTeffes 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 lead 

 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, acting 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 correct the colour, namely, 

 three wedges of crown-glafs, to one of the butter of anti- 

 mony. 



This 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 eye-glafs, 

 that this fluid ; in its Highly concentrated flate, was fubject, like 



flint- 



