Ahirratkn from Colour totally rcmoveJ. 1 1 



retl fringe, which is the prevailing colour of the leaft refrangible rays ; and if the eyc- 

 glafs be placed at a diftance beyond that which is required for dillinct vilion, it will be 

 furrounded with a blue fringe, which is the prevailing colour of the nioft refrang'ble rays. 



The reafon of this will appear more clearly from infpedling the fixth figure, where the 

 red rays appear outermoil within the focus at A, and the violet rays appear outermod be- 

 yond tlv; focus at B. Thele colours arc alfo vifible when an image of any luminous object, 

 as the fun, is formed by a lens on a white ground ; and they will be fo much the more 

 confpicuous, the greater the diameter of the lens in proportion to its focal diftance. The 

 reverfe of this happens in compound lenfes, when the medium employed to correcl the 

 colour difperfes more than it ought to do. 



In this way the correction of the colour may be examined, and the qualities of refrafting 

 mediums iiiveftigated, to an extreme degree of accuracy. Yet the effe£l will be rendered 

 ftill more fenfible by covering half the objecl-glafs. For, when this is done, the colour pro- 

 duced by the uncovered half of the obje£l-glafs appears, without being mixed with that of 

 the oppofite fide, even vrhen the eye-glafs is adjufted to diftincl vifion. Thus, in Fig. 6, the 

 colours produced by both fides of the lens are mixed at the general focus F. But if the 

 rays coming from one fide be intercepted, thofe which are refraded by the other fide will 

 appear in their proper colours. By thefe riieans, and by employing a very luminous object 

 furrounded by a dark ground, and a high magnifying power, the leall un>."jrre£ted colour 

 may be rendered fenfible. 



The firft conjefture which prefented itfelf on the obfervation of irregularities of this 

 kind, in compound lenfes adjufted fo as to correfl the aberration of the extreme rays, was, 

 that the colour might fomehow proceed from the compenfation at different diftances from 

 the axis of lenfes not correfponding as the plane furfaces of prifms everywhere do. Trials 

 on the images formed by the central parts, and afterwards by the parts near the circum- 

 ference, proved, however, that this conjedlure did not reach the caufe. Subfcquent expe- 

 riments and examinations of the coloured fringes led the author to the true caufe, that is 

 to fay, the want of proportionality in the difperfive powers of the different mediums, as has 

 been already ftated. It remained, theiefore, to difcover, by a new procefs of experimental 

 enquiry, the adequate remedy for fuch an extreme fource of imperfection. This led to the 

 valuable doctrine, that one clafs of difperfive mediums throw the green nearer to the violet, 

 while another clafs throw the fame colour nearer to the red, than is feen in the fpeflrum 

 formed by crown-glafs and other mediums, which difperfe but in a fmall degree. Thefe 

 mediums were therefore applied in oppofition to each other, to correit this fecondary 

 aberration in lenfes more compounded than might have been required if fuch a difficulty had 

 not prefented itfelf. And laftly, the gieatcr degree of compofition in thcfo pcrfeft knfes 

 was got rid of by the happy expedient of mixing the two different Icinds of difperfive me- 

 diums, which, as it fortunately turned out, did produce a compofition of an intermediate 

 nature, with regard to this proportionality in the arrangement of the prifmatic colours. 



It has already been remarked, in the order of this abridgement, that, in metallic folutioiis 

 and cffcntial oils, the green light is among the lefs refrangible rays; but that, in the marine 

 and nitrous acids, the fame green light becomes one of the more refrangible. It was not 

 probable that the cffcntial oiU could be united with marine acid fo as to form a colouilcfs 



C 2 fluid 



