^ii On the Powers (if the fnjmat'ic CotdurS 



been mentioned in the 6th and 7th experiments, I was vvifling 

 to try whether they proceeded from this mixture ; and there- 

 fore covered the prifm in front with a piece of pafteboard, 

 having a (lit in it of about one-tenth of an inch broad. 



10th Ex[ertment. The naii at 9 feet 2 inches from the 

 primi.^ — R. I fix my attentian on two fliining red points; 

 they are pretty bright. — 0. I fee many more points j the 

 objeft is better illuminated than in the red ; the points are- 

 furrounded by black, but afe orange-eoloured. — Y. The 

 points naw are yellow, and white furrounded by black; the 

 objeft is better illuminated than in orange. The maximimt 

 of illumination is in the brighteft yellow or paleft green. — 

 O4 The points are green and white, as before furrounded by 

 black; better illuminated than in orano;e.— jB. The illumf- 

 nation is nearly erjn^? to red. — -I. Very indifferently illumi- 

 Hated. — F. \\'\-y badly illuminated. 



The phcenomcna of the differently coloured points being 

 flow completely refulved, fince they were plainly owing to 

 the former admixture of colours, and the iRuminating power 

 remaining afcertained as before, I attempted alfo to repeat 

 the experiments upon the thermometer, with the prifm co- 

 vered in the fame manner;; but I found the efre<ft of the 

 coloured rays too much enfeebled to give a decifive refult. 



I might now proceed to my next fubj€6t; but it may be 

 pardonable if I digrefs for a moment, and remark that the 

 foregoing refearches ought to lead us on to others. May 

 not the chemical propcftics of the prifmatic colours be as 

 diiFerent as thofe which relate to light and heat ? Adequate 

 ftiefhods for an invefligation of them may eafily be found; 

 and we cannot too minutely enter into an analyfis of light, 

 which is the moft fubtile of all the aftive principles that are 

 concerned in the mechanifm of the operations of nature. 

 A better acquaintance with it may enabie us to account for^ 

 various fa6ls that fall under our daily obfervation, but which 

 have hitherto remained unexplained. If the power of heat- 

 ing, as we flow fee, be chiefly lodged in tht red-making. 

 rays^ it accounts for the comfortable warmth that is thrown 

 out from a fire, when it is in the ftateof a red glow ; and for 

 the heat which is given by charcoal, coke, and balls of fmall- 

 7 «oal 



