S2 On the UNEQUAL 



lefs tranfparent fluid. But nothing can be better adapted for 

 this purpofe than metallic folutions. 



I first made trial of butter of antimony, and found the 

 refult to be what I expected. On increasing the proportion of 

 muriatic acid, the fringes of green and purple grew narrower 

 and narrower till they entirely difappeared, and if more was 

 then added they re-appeared in an inverted order. I tried the 

 fame thing with a folution of crude fal ammoniac and mercury 

 fublimate. If the folution contains a certain proportion of 

 thefe two fubflances, the rays of all colours emerge from the 

 compound object-glafs equally refracted. If the proportion of 

 the ammoniacal fait, and confequently of the muriatic acid 

 which it contains, be increafed, the green rays, which were 

 the mean refrangible in the difperfive fluid, as well as in crown- 

 glafs, draw nearer to the violet, making a part of the more re- 

 frangible half of the fpectrum, and confequently emerge lefs 

 refracted than the united red and violet rays, and are converged 

 to a focus at a greater diflance from the object-glafs ', fo that 

 the green fringe now appears within the focus, and the purple 

 fringe beyond it. But on increafing the proportion of mercu- 

 rial particles, thefe fame green rays fhift their fituation to the 

 lefs refrangible half of the fpectrum, which appears from their 

 now emerging moft refracted, and being converged to a point 

 nearer to the object-glafs than the united red and violet, whofe 

 refrangibility does not appear to be affected by thefe admixtures 

 which occafion fuch remarkable fluctuations in the refrangibi- 

 lity of the green rays and other intermediate orders. It may 

 poflibly feem ftrange at firft view, that the green rays fhould 

 emerge moft refracted from the compound object-glafs, when 

 their refrangibility in the difperfive medium is diminifhed, 

 and leaft refracted under the contrary circumftances. The 

 caufe of this is, that the principal refraction of the compound 

 object-glafs is performed by the indifperfive convex lens, 



which 



