REFRANGIBILlTr of LIGHT. 33 



yond the focus at B. Thefe colours may alfo be feen, when 

 an image of any luminous object, as the fun, is formed bv a 

 lens upon a white ground ; and they will be fo much the more 

 confpicuous, by how much the diameter of the lens is greater, 

 in proportion to its focal diftance. 



Just the reverfe of this will happen in a compound object- 

 glafs, if, in correcting the colour, the medium employed dif- 

 perfes more than it ought to do. A blue fringe will then ap- 

 pear round a luminous object, when the eye-glafs is pufhed in ', 

 and a red fringe, when it is drawn out beyond what is necefTary 

 for diflinct vifion. 



In this way, the correction of the colour may be examined, 

 and the qualities of refracting mediums inveftigated, to an ex- 

 treme degree of accuracy ; yet the effect will be rendered dill 

 more fenfible, by covering half the object-glafs. For when 

 this is done, the colour produced by the uncovered half of the 

 object glafs appears, without being mixed with that of the op- 

 pofite fide, even when the eye-glafs is adjufted to diflinct vi- 

 fion. Thus, in Fig. 13. the colours produced by both fides of 

 the lens, are mixed at the general focus F. But if the rays co- 

 ming from one fide be intercepted, thofe which are refracted 

 by the other fide will appear in their proper colours. By thefe 

 means, and by employing a very luminous object, furrounded 

 by a dark ground, and a high magnifying power, the lead un- 

 corrected colour may be rendered fenfible. 



My firft obfervations, which clearly proved the correction of 

 colour which is obtained by the combination of two mediums 

 differing in difperfive power, to be only partial, were made in 

 the fummer of the year 1787, at Merchifton. 



I had, fome time before, found it impofhble to fucceed, in P 



this refpect, with prifms compofed of crown and flint glafs. 

 But as 1 neither was able to make the phenomena fo apparent 

 by this method as with lenfes, nor had a command of prifms 

 with that great variety of refracting angles necefTary to put it 



Vol. III. E. beyond 



