513 On the Powers of the prifmatlc Colours 



of what was before admitted upon truft may become of great 

 utili y to natural knowledge. Thus, for inflance, when we 

 fee the effeA of the condcnfatiou of the fun's rays in the 

 focus of a burning lens, it fecms to be natural to fuppofe 

 that every one of the united rays contributes its proportional 

 Iharc lo the intenfity of the heat which is produced; and we 

 fiiould probably think it highly abfurd, if it were alTerted 

 that m.mv of them had but little concern in the combulHon 

 orvilriricaiion which follows, when an obje6l is put into that 

 focus. It will therefore not be amifs to mention what gave 

 rife to a furmife, that the power of heating and illuminating 

 objects might not be equally diitributed among the varioufly 

 coloured ravs. 



In a variety of experiments which I have occafionally made^ 

 relating to the method of viewing the fun with large tele- 

 fcopes to the bed advantage, I ufed various fcombinationS 

 of ditferenlly- coloured darkening glaflcs. What appeared 

 remarkable was, that when I ufed fome of them I felt a fenfa- 

 ♦ion of heat, though I had but little light; while others gave 

 me much light, with fcarce any fenlation of heat. Now, 

 as in thefe different combinations the fun's image was alfo 

 differently coloured, it occurred to me, that the prifmatie 

 rays might have the power of heating bodies very unequally 

 diftributerl among them ; and, as I judiied it right in this 

 refpeft to entertain a doubt, it appeared equally proper to 

 admit the lame with regard to light. If certain colours 

 fliotild be more apt to occafion heat, others might, on the 

 contrary, be more fit for vifion, by poffeffing a fuperior illu- 

 minating power. At all events, it would be proper to recur 

 to e.Kperiments for a dccifion. 



Experhnerits on the heating Poiccr of coloured tlays. 



I fixed a piece of pafleuoard, A6, (Plate IX.) in a frame, 

 iriounted upon a Itand, CD, and moveable upon two centres. 

 In the padeboard I cut an opening, mn, a little larger thari 

 the ball of a thermometer, and of a fufficient length to let 

 the whole extent of one of the prifmatie colours pafs through. 

 I then place.! tbr-je thermometers upon fmali intlined planesy 



