VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 675 



lar effects. It has been explained, 1st. Why air heated by the moderate warmth 

 of the sun's rays, occasions objects to appear doubled and inverted. 2dly. Why 

 rarefaction, by a higher degree of heat, gives an additional image, which is not 

 inverted. 3dly. In what state of evaporation the increase of the air's density 

 brings distant objects into view by unusual elevation. 4thly. Under what circum- 

 stances evaporation may also produce an inverted image less elevated. And it is 

 probable, that the same reasoning will afford a ready explanation to other varieties 

 of terrestrial refraction, that may have been, or may hereafter be observed. 



XIII. Investigation of the Powers of the Prismatic Colours to Heat and Illuminate 

 Objects ; with Remarks, that prove the Different Refrangibility of Radiant Heat, 

 To which is added, an Inquiry into the Method of Viewing the Sun advan- 

 tageously, with Telescopes of Large Apertures and High Magnifying Powers. 

 By Wm. Herschel, LL. D., F. R. S. p. 255. 



It is sometimes of great use in natural philosophy, to doubt of things that are 

 commonly taken for granted ; especially as the means of resolving any doubt, when 

 once it is entertained, are often within our reach. We may therefore say, that any 

 experiment which leads us to investigate the truth of what was before admitted on 

 trust, may become of great utility to natural knowledge. Thus, for instance, 

 when we see the effect of the condensation of the sun's rays in the focus of a 

 burning lens, it seems to be natural to suppose, that every one of the united rays 

 contributes its proportional share to the intensity of the heat produced ; and we 

 should probably think it highly absurd, if it were asserted that many of them had 

 but little concern in the combustion, or vitrification, which follows, when an object 

 is put into that focus. It will therefore not be amiss to notice what gave rise to a 

 surmise, that the power of heating and illuminating objects, might not be equally 

 distributed among the variously coloured rays. 



In a variety of experiments I have occasionally made, relating to the method of 

 viewing the sun, with large telescopes, to the best advantage, I used various com- 

 binations 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 co- 

 loured, 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 re- 

 gard to light. If certain colours should be more apt to occasion heat, others 

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

 nating 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 paste- 

 board, ab, pi. u, fig. 13, in a frame mounted on a stand, CD, and moveable on-2 



4B2 



