52® On the Powers of the pr-ifmntic Cohurs 



illuminated that I cannot fee the object; or at leaft but 

 tarely perceive that it exifts. 



gth Experunent. Black paper at 8 feet from the prifm.— - 

 i?. The objeiSl is hardly vifible ; I can onlv fee a few faint 

 points.-T— 0. I fee feveral bright points and many faint ones. 

 — T. Numberlefs bright and fmall faint points; between 

 yellow and green is the maximum of illumination. — G. The 

 fame as the yellow. — B. V'ery indifierently illuminated; but 

 not {o bad as in the red rays, — I. I cannot fee the objeft, — 

 V. Totallv inviilble. 



From thcfe obfervations, which a<rrce uncommonly well 

 with refpecl to the illuminating power affigned to each co- 

 lour, we inay conclude that the red-making rays are very 

 far from having it in any eminent degree. The orange pof- 

 fcfs more of it than the ved; and the yellow ravs illuminate 

 objeds ftill more perfeftly. The maximum of illumination 

 lies in the brighteft yellow, or paleft green. The green itfelf 

 is nearly equally bright with the yellow; but, from the full 

 deep green, the illuminating power decreafes very fenfibly. 

 That of the blue is nearly upon a par with that of the red ; 

 the indigo has much lefs than the blue ; and the violet is 

 very deficient. 



With regard to the principle of dirtin(5lnefs, there appears 

 to be no deliciency in any one of the colours. In the violet 

 rays, for inftance, fome of the experiments mention that 1 

 faw badly; but this is to be underftood only with r<fpe6l to 

 the numtjer of fmall objefts that could be perceived; for, 

 although I faw fewer of the points, thofe whi'-h remained 

 yifihle were always as diCtinol as, in fo feeble an illumination, 

 could be expected. It muft indeed be evident that, by re- 

 moving the great obfiacie to diltinft vifion, which is the dif- 

 jerent refrangibility of the rays of light, a microfcope will be 

 capable of a much higher degree of diftinclnefs than it can 

 he uinder the ufual circum dances. A celebrated optical 

 ■u. liter has formerly remarked that a fly, illuminated bv red 

 rays, appeared uncommonly diftinft, and that all its minute 

 parts might be feen in great perfeftion ; and, from the ex- 

 j?eriments which have been related, it appears that every 

 other colour is pofTclTcd of the fame advantage. 



