- REFRANGIBILITY of LIGHT. 7 
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 fingular and important quality was deteéted 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 fpeftrum, 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. 
Tuis remarkable quality found in the marine acid led to 
complete fuccefs in removing the great defe& of optical inftru- 
ments, that diflipation or aberration of the rays, arifing from 
their unequal refrangibility, which has rendered it impoffible 
hitherto to converge all of them to one point, either by fingle 
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 raysof the {pectrum 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 refraGtion which takes place in 
the confine of it and glafs, may be rendered as regular as re- 
fiction, while the errors arifing from unavoidable imperfec- 
tions of workmanthip, are far lefs hurtful than in reflection, 
and 
