268 Achromatic Telefctsps. [Book HI. 



degree of refrangibility than the red rays, they muft of 

 confequence converge more by the fame power of re- 

 fraction in the glafs, and meet fooner in a focus, fup- 

 pofe at r. Let now the concave lens dd be placed in 

 fuch a manner as to intercept all die rays before they 

 corne to their focus. If this lens was made of the fame 

 materials, and ground to the fame radius with the 

 convex one, it would have the fame power to caufe 

 the rays to diverge that the former had to make them 

 converge. In this cafe, the red rays would become 

 parallel, and move on in the line 00,00: but the con- 

 cave lens, being made of flint glafs, and upon a fhorter 

 radius, has a greater refractive power, and therefore 

 they diverge a little after they come out of it; and if 

 no third lens was interpofed, they would proceed di- 

 verging in the lines opt, opt-, but, by the interpofi- 

 tion of the third lens ovo, they are again made to con- 

 verge, and' meet in a focus fomewhat more diltant than 

 the former, as at x. By the concave lens the violet 

 lays are alfo refracted, and made to diverge : but hav- 

 ing a greater degree of refrangibility, the fame power 

 of refraction makes them diverge fomewhat more than 

 the red ones; and thus, if no third lens was interpofed, 

 they would proceed in fuch lines as Imn 3 Imn. Now 

 as the differently coloured rays fall upon the third lens 

 with different degrees of divergence, it is plain, that 

 the fame power of refraction in that lens will operate 

 upon them in fuch a manner as to bring them all to- 

 gether to a focus very nearly k at the fame point. The 

 red rays, it is true^ require the greateft power of re- 

 fraction to bring them to a focus , but they fall upon 

 the lens with the lead degree of divergence. The 

 violet rays, though they require the lead power of re- 

 fraction, yet have the greateft degree of divergence j. 



and 



