REFRANGIBILITY of LIGHT. 43 
rendered as colourlefs as poflible, I began by trying other dif- 
perfive mediums, which owe this property to different metallic 
or faline particles with which they are impregnated, in hopes 
of finding fome difperfive medium, which might feparate the 
differently refrangible rays in the fame proportion in which 
crown-glafs does, and thus afford a method of refraéting all of 
them alike, and confequently without colour. But I was dif- 
appointed. The compound object-glafles, formed of a variety 
of difperfive fluids and crown-glafs, exhibited green and purple 
fringes, as before, which proved the difperfive power of the 
two mediums not to be proportional. : 
My next ftep was to vary the combination by rejecting glafs 
entirely as a refracting medium, and only employing it to con- 
fine the fluids. As a fluid medium was here to be ufed as a 
convex lens, thofe which had been found leaft difperfive, were 
to be made choice of. Accordingly, water, {pirit of wine, ni- 
trous and vitriolic ether, and all the limpid indifperfive fluids 
I could come at, were made trial of. But ftill the refult was 
the fame. The green and purple fringes appeared, | on covering 
half the object-glafs.. 
‘I ruen fubftituted fome other of the more perfect indifper- 
five kinds of glafs inftead of crown-glafs ; but with no better 
fuccefs. 
- Next I combined two effential ‘oils, both of them more dif- 
perfive than crown-glafs, but differing fo confiderably in this - 
refpect between themfelves, that the lefs difperfive could be 
ufed as a convex, while the other was fo difpofed as to perform 
‘the office of a concave. For it will eafily be underftood, that 
lenfes of any kind may be formed of fluid mediums, by in- 
cluding them between glaffes, which have one fide formed 
convex, and the other concave, to the fame radius, and thus 
ferve merely to confine the fluids, without producing themfelves 
any effe&t in refracting the light. If a flat fide is wanted, a 
Her of plain glafs with parallel fides muft be ufed, and in 
B2 concave 
