REFRANGIBILlTr of LIGHT, Ji 



pound concave lens of the materials which produce moft colour, 

 and combining it with a compound convex lens formed of the 

 materials which produce leaft colour ; and it was obferved in 

 what manner this might be effected by means of three mediums, 

 though apparently four are required. 



In fearching for mediums beft adapted for the above purpofe, 

 a very Angular and important quality was detected in the mu- 

 riatic acid. In all the difperfive mediums hitherto examined, 

 the green rays, which are the mean refrangible in crown- glafs, 

 were found among the lefs refrangible, and thence occafion the 

 uncorrected colour which has been defcribed. In the muriatic 

 acid, on the contrary, thefe fame rays make a part of the more 

 refrangible ; and in confequence of this, the order of the co- 

 lours in the fecondary fpeclrum, formed by a combination of 

 crown-glafs with this fluid, is inverted, the homogeneal green 

 being now the leaft refrangible, and the united red and violet 

 the moft refrangible. 



This remarkable quality found in the marine acid led to 

 complete fuccefs in removing the great defect of optical inftru- 

 ments, that diflipation or aberration of the rays, arifing from 

 their unequal refrangibility, which has rendered it impoflible 

 hitherto to converge all of them to one point, either by Angle 

 or oppofite refractions. A fluid in which the particles of ma- 

 rine acid and metalline particles hold a due proportion, at the 

 fame time that it feparates the extreme rays of the fpectrum much 

 more than crown-glafs, refracts all the orders of rays exactly in 

 the fame proportion as the glafs does ; and hence rays of all co- 

 lours, made to diverge by the refraction of the glafs, may ei- 

 ther be rendered parallel by a fubfequent refraction made in the 

 confine of the glafs and this fluid, or by weakening the refrac- 

 tive denfity of the fluid, the refraction which takes place in 

 the confine of it and glafs, may be rendered as regular as re- 

 flection, while the errors arifing from unavoidable imperfec- 

 tions of workmanfhip, are far lefs hurtful than in reflection, 



and 



