REFRANGlBILITr of LIGHT. 45 



with glafs. Hence I was led to conclude, that if I took an 

 achromatic convex lens, compofed of the two eifential oils, and 

 combined it with an achromatic concave lens of a longer focal 

 diftance, compofed of crown-glafs and either of the effential 

 oils, I fhonld be able, through fuch a double compound object- 

 glafs, to converge the rays to a focus, without any aberration 

 whatever from the difference of refrangibility of light. For if 

 the compound convex and compound concave are properly pro- 

 portioned to each other, the fecondary fpectrums, or fringes of 

 green and purple, may be rendered of the fame breadth in both 

 lenfes \ and from the obfervations before related, this will hap- 

 pen when there is a considerable balance of refraction in favour 

 of the convex lens. For it is compofed of materials which 

 form a much narrower fecondary fpeclrum, under an equal re- 

 fraction of the whole pencil, than thofe mediums do, of which 

 the compound concave is formed. 



This will be underftood, by attending to what takes place 

 in the refractions of light through the lenfes, without again re- 

 curring to the more fimple cafe of prifms. 



Fig. 17. reprefents a compound concave lens, formed of a 

 concave lens of glafs, and a concave lens of a difperfive fluid, 

 but of a fhorter focus than the concave lens, and fo proportion- 

 ed as to produce a refraction as free from colour as can be ob- 

 tained by a combination of thefe two mediums. This lens 

 being expofed to parallel rays, will make them diverge, after re- 

 fraction, from its virtual focus, and the united red and violet 

 rays will be the lead refracted, and will be inclined in a certain 

 angle to the green rays which are moft refracted, as reprefented 

 in the figure. 



Fig. 18. reprefents a compound convex lens, formed of 

 a convex of an effential oil, which difperfes the rays in 

 a lgffer degree, combined with a concave of an effential oil, 



which 



