4 
ga On the UNEQUAL 
lefs tranfparent fluid. But nothing can be better adapted for 
this purpofe than metallic folutions. 
I rirst made trial of butter of antimony, and found the 
refult to be what I expected. On increafing 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 fubftances, the rays of all colours emerge from the 
compound objeét-glafs equally refracted. If the proportion of 
the ammoniacal falt, 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 diftance 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 fpe€trum, which appears from their 
now emerging moft refracted, and being converged to a point 
nearer to the objeét-glafs than the united red and violet, whofe 
refrangibility does not appear to be affected by thefe admixtures 
which occafion fuch remarkable flu€tuations 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 refraéted under the contrary circumftances. The 
caufe of this is,-that the principal refraétion of the compound 
object-glafs is performed a the indifperfive convex lens, 
which 
t 
