42 On the UNEQUAL 
the menifcus, and the vacancy between them is replenifhed by 
a fluid of the requifite degree of difperfive power, which is 
confined by means of a ring of glafs. Thefe lenfes are two 
inches and feven eighths of an inch in diameter, and the focal 
length of the compound objedt-glafs is ten inches; the curva- 
tures of the lenfes being fo proportioned, as nearly to correct 
the aberration from the fpherical figure. 
Tue fluid I employed to remove the colour arifing from the 
different refrangibility of light, was an effential oil, whofe dif- 
perfive quality I could eafily increafe or diminifh, by mixing it 
with others differing in their difperfive qualities, though of 
nearly the fame mean fefra¢tive denfity ; by which means, the 
correction of the error from the figure was not difturbed, by 
varying the ftrength of the difperfive fluid. I now expeéted 
perfectly to remove the colour, by adding a little of one or 
other of the difperfive fluids, as occafion might require. 
Berore relating the event of this trial, it will be proper to 
explain the manner of examining the difperfive power of fluids 
by means of lenfes, and of Gaeta: when the colour is 
perfectly corrected. 
Wuen the image of a lucid point is formed { in the focus of a 
fimple lens, the violet rays are converged to a focus neareft to 
the lens, and the deep red rays are converged to a focus at the 
greateft diftance from it. The confequence of this is, that if 
this image be examined by an eye-glafs nearer to the lens than 
is required for diftiné vifion, it will appear furrounded with a 
red fringe, which is the prevailing colour of the leaft refrangi- 
ble rays ; and if the eye-glafs be placed at a diftance beyond 
that which is required for diftiné vifion, it will be furrounded 
with a blue fringe, which is the prevailing colour of the moft 
refrangible rays. 
Tue reafon of this will appear more clearly from infpecting 
the thirteenth figure, where the red rays appear outermoft with- 
in the focus at A, and the violet rays appear outermoft be- 
yond 
