REFRJNGIBILirr 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 refracting all of 

 them alike, and confequently without colour. But I was dif- 

 appointed. The compound object-glaffes, 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, fpirit of wine, ni- 

 trous and vitriolic aether, 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 then fubflituted fome other of the more perfect indifper- 

 five kinds of glafs inftead of crown-glafs ; but with no better 

 fuccefs. 



Next 1 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 underflood, 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 effect in refracting the light. If a flat fide is wanted, a 

 piece of plain glafs with parallel fides muft be ufed, and in 



F 2 concave 



