256 Dr. Herschel's Investigation of the Powers 



to the method of viewing the sun, with large telescopes, to the 

 best advantage, I used various combinations of differently- 

 coloured darkening glasses. What appeared remarkable was, 

 that when I used some of them, I felt a sensation of heat, 

 though I had but little light ; while others gave me much light, 

 with scarce any sensation of heat. Now, as in these different 

 combinations the sun's image was also differently coloured, it 

 occurred to me, that the prismatic rays might have the power of 

 heating bodies very unequally distributed among them ; and, as 

 I judged it right in this respect to entertain a doubt, it appeared 

 equally proper to admit the same with regard to light. If cer- 

 tain colours should be more apt to occasion heat, others might, 

 on the contrary, be more fit for vision, by possessing a superior 

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

 to experiments for a decision. 



Experiments on the heating Power of coloured Rays. 



I fixed a piece of pasteboard, AB, ( Plate X.) in a frame, mounted 

 upon a stand, CD, and moveable upon two centres. In the 

 pasteboard, I cut an opening, mn, a little larger than the ball of 

 a thermometer, and of a sufficient length to let the whole extent 

 of one of the prismatic colours pass through. I then placed 

 three thermometers upon small inclined planes, EF : their 

 balls were blacked with japan ink. That of No. 1 was rather 

 too large for great sensibility. No. 2 and 3 were two excellent 

 thermometers, which my highly esteemed friend Dr. Wilson, 

 late Professor of Astronomy at Glasgow, had lent me for the 

 purpose : their balls being very small, made them of exquisite 



