39© On the Number of the Primitive Colorific Rays in Solar Liglt. 



equally refrangible, they will be alfo equally refle>:ib!e ; and therefore cannot be feparated 

 either by refraction or refledtion, fo as to exhibit the different coloured rays of which they 

 are compofcd. It feems therefore, that the only way remaining, by which we can expe- 

 rimentally afcertain the compoGtion of thefe colours, if they be indeed compound, is tranf- 

 -miffion. For fince tranfparent coloured bodies are fuch merely by their letting pafs through 

 them either folely, or more copioufly, rays of a certain colour, and intercepting all others, 

 fuch tranfparent bodies, applied to compound colours, will afcertain that compofition, by 

 extinguifliing, in a great meafure, all rays except fuch as are fo adapted to its conforma- 

 tion, as to pafs through it, and give it its peculiar denomination of colour. 



In order to try the truth of the hypothefis of feven colours by this teft, I looked through 

 a blue glafs at the red end of the fpe£lrum : now we are to confider, that if that part of the 

 fpeftrum was compofed of red rays, and none other, the only effedl: of the blue glafs would 

 cither be a total or partial fuffocation of thi red rays j and therefore that part of the fpec- 

 trum, when looked at through the glafs, would either totally difappear, or become a faint 

 and diluted red. But, on experim£nt it appeared of a purple colour. The purple in this 

 cafe could not be a primltive^nd original colour, as is manifeft, becaufe it did not proceed 

 from the purple part of the fpeftrum ; we muft therefore conclude, that it was a compound 

 ■ colour. But purple, when compound, is made up of blue and red, therefore it follows, 

 that fome blue rays did a£tually exift in the red part of the fpedlrum : which combined 

 with the few, ftraggling red rays which penetrated the blue glafs, compofed ' that purple 

 colour, which the red extremity of the fpedrum affumed, when viewed by the light tranf- 

 mitted through the blue medium. 



To try, ou the other hand, whether any red rays lay hid amongft the blue,' I proceeded 

 In the fame manner, and looking at the blueft part of the fpedtrum through a red glafs, it 

 appeared of a purple colour : fome red rays therefore are equally refrangible with the 

 blue i and if the red extends as far as the blue, there is no reafon why we may not fuppofc 

 that it extends fomewhat farther, fo as to compound, with a diluted blue, the extreme 

 colours of the fpedlrum, Indigo and violet. 



But it may be faid, that if blue rays exifted amongft the red, that part of the fpe£l;rum 

 could not appear fo extremely brilliant as it really does ; but would put on a purplifh ap- 

 pearance in the fpe£lrum itfelf, even to the naked eye. In anfwer to this objeftion we may 

 obferve, that the moft intenfe and vivid, natural red bodies do, in fad, refleft a very great 

 proportion of blue rays, beCaufe they appear of a ftrong blue colour when placed in the 

 blue part of the fpedrum ; and therefore they refledt juft as many when the dire£t, white 

 folar light falls on them, in which all that blue is involved ; though by the predominance 

 of the red rays, they appear of that colour, without any vifible tindure of blue. 



In order to determine whether the purple appearance of the red extremity of the fpec- 



trum, when viewed through a blue glafs, was caufed by any of the white folar ligKt, which 



might perhaps be reflefted from the air, or furrounding objeds to the fpeftrum, and thus 



Uirow on that part fuch a quantity of blue as might produce a fenfible effe£l; ; I caufed the 



4 middle 



