On the Number of the Primitive Colorific Rays in Solar Light: 391 



middle and moft intenfe part of the red to pafs through a hole in a blackened paper, and 

 then fall on an optical fcreen ; by which I was fure that I had as pure and uncompounded 

 a red as could be defired ; which alfo underwent the ufual teft of purity by fubfequent re- 

 fradion, without any change in the form of the fpeftrum ; I then looked at the body 

 which was illuminated with this red, through the fame blue glafs, and the eiFeft was the 

 fame as before. 



To try this do£lrine of three parent colours ftill farther, I confidered, that if the orange 

 were really compounded of the red and yellow rays, then by looking at the orange through 

 a red glafs, the orange would in a great meafurc vanilh, and the red would appear to ex- 

 tend much farther than in the original fpe£trum ; becaufe the yellow rays being confider- 

 ably obftrufted, the red would become more predominant ; and that part of the fpe£trum, 

 which before appeared orange, in confequence of a certain mixture of yellow and red, 

 would now, by the failure of fo confulerable a part of the yellow, lofe its orange appear- 

 ance, and put on that of red: and, on experiment, I found the cafe to be fo really in fa£l ; 

 for while an afliftant looked at the fpedlrum through the red glafs, I moved an obftacle 

 from the red towards the other end of the fpe(Srum, dcfiring him to flop me, when the 

 obftacle fhoukl arrive at the confines of red and orange ; but when he did fo, the obftacle 

 had attained the middle of the orange, or rather had pafled beyond it. Now if the orange 

 were really a primitive colour, I fhould fuppofe, that when looked at through the red glafs, 

 it would either appear diluted, without any change of dimenfions; or that if the weak part 

 of the orange, next the red, ftiould vanifli, by the obftrudipn of the glafs, a dark Interval 

 would appear between the orange and the red} in neither cafe can we account for the 

 apparent extenfion of the red into the region of the orange •, nor by any other hypothefis, 

 as appears to me, than that fome of the red rays are equally refrangible with fome of the 

 orange. 



There is another argument derived from the ocular fpeftra of Dr. Darwin, which ftill 

 further corroborates the doftrine of three primogenial colours. Place a p'ece of coloured 

 filk, about an inch in diameter, on a ftieet of white paper, about half a yard from your 

 eyes ; look fteadily upon it for a minute ; then remove your eyes upon another part of the 

 white paper, and a fpeftrum will be feen of the form of the filk thus infpeded, but of a 

 different colour, thus 



Red filk produced a green fpe£lrum. 



The reafon of thefe phsenomena is very ingenioufly afligned by Dr. Darwin; he fay?, 

 that the retina being excited into a violent and long continued, action by the red rays, in 

 the firft experiment, at length is fo fatigued as to become infenfible to themj but that it 



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