%1 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 objecVglafs is ten inches ; the curva- 

 tures of the lenfes being fo proportioned, as nearly to correct 

 the aberration from the fpherical figure. 



The fluid I employed to remove the colour ariflng from the 

 different refrangibility of light, was an efTential 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 refra<t ive denfity ; by which means, the 

 correction of the error from the figure was not diflurbed, by 

 varying the flrength of the difperfive fluid. I now expected 

 perfectly to remove the colour, by adding a little of one or 

 other of the difperfive fluids, as occafion might require. 



Before 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 diflinguifhing when the colour is 

 perfectly corrected. 



When 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 diflinct vifion, it will appear furrounded with a 

 red fringe, which is the prevailing colour of the leafl refrangi- 

 ble rays ; and if the eye-glafs be placed at a diftance beyond 

 that which is required for diflinct vifion, it will be furrounded 

 with a blue fringe, which is the prevailing colour of the mofl 

 refrangible rays. 



The 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 



