Inquiries into Coloicred Light, T09 



through whicli the fpe^lrum is viewed, decreafes, in any con- 

 fiderable deirree of its briohtncfs, there will come on blueidi 

 hues, and a very faint intervening greenifh une. To try this 

 circmnliancc iiiore decidedly, 1 threw up the iafli, and made 

 xjn affillant hold a broad rule, fo that I might view the Ipec- 

 trum as it proceeded from iiie two edges in the fame manner 

 as from the frames of the fulh : I then direded my aflidant 

 to lower the rule down to the bottom of the window, till the 

 yellow in the under gradations interiefted the bottom. The 

 yellow of this polition not going off into tranfparent light, 

 but into Ihade, the blue hues appeared in very decided tints 

 through all their gradations ; and where blue, fucceeding to 

 the yellow, intcrfccted it, a decided ercen intervened. 



Althou^Ii no conuDon eye, unprafiifed in the ufe and ar- 

 rangement of colours and their various tints, can difcover m 

 the fpaces of the prifmatic colours called homogeneous, whcu 

 thrown on an opaque furface, any gradations of tints; yet 

 when thefe colours are viewed by the prifm through an opeu 

 light fo as to be tranfparent, the gradation of fuceeflivc tints 

 in each homogeneous fpace becomes evidently obfervable to 

 the moil connnon eye. The obferver may difccrn the fa<ft, 

 that no one of thofe prifmatic colours which are called ho- 

 mogeneous is abfohitely fo, but that, as according to the 

 theorem ftated by Sir Ifaac Newton himfclf, they confift of 

 a fuccedive ferics of innumerable circular or orbicular image* 

 of the fun ; the tints of the commonly called homogeneous 

 colour, red, vary according to the innuniL-rable fucceeding 

 ancles of refraction, heightening gradually towards orangey 

 and going gradatim into the orange; and that the orange 

 heightens in the fame gradual fucceflion of tints going into 

 yellow, and the yellow, in like manner, into pure light : 

 alfo that the blues of the other gradation of colours are .an 

 indifcriminate fucccdion of tints or hues. 



Conformably lo what \r> here ftated of the heightening of 

 the irradations of the colours at one end of the fptctrum into 

 light ; of tlie blues going off, by a like gradali<m at the other 

 end, into fliade or dark; Sir Ilaac Newton (Optics, book i. 

 prop. V. exper. j6. p. 85.) llales, that " the molt lumhioiis 

 of the prifmatic colours are the orange and the yellow ; ne\t 

 the f^reen ; and that blue is a faint and dark colour, and the 

 indii^o and violet the weakelt and darkclt of all colours." 



Now from a more decifive examinatiorl of the fatSl by 

 a^lual experiment we fli^dl be led to lee demonllrably that 

 thefc ai)i)arciu hues, which arc called blue colours, art; not 

 colour, out a mere moditication of light going olV mlu (hade, 

 u ^iirt'uil and Jtucejjhc i!rprhali',7i 0/ lig/t. 



In 



