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Investigation of the Powers of the prismatic Colours to heat and illu- 

 minate Objects ; with Remarks, that prove the different Refrangi- 

 bility of radiant Heat. To which is added, an Inquiry into the Me- 

 thod of viewing the Sun advantageously, with Telescopes of large 

 Apertures and high magnifying Powers. By William Herschel, 

 LL.D. F.R.S. Part I. Read March 27, 1800. [Phil. Trans. 

 1800, p. 255.] 



After recommending a cautious circumspection in admitting spe- 

 cious appearances and plausible inferences in our researches both 

 after physical and moral truth, the Doctor acknowledges that a general 

 diffidence of this nature had raised a doubt in his mind, that the 

 power of heating and illuminating objects is not equally distributed 

 among the various coloured rays. This surmise received some con- 

 firmation from the different sensations he experienced on viewing the 

 sun with his large telescopes, and through various combinations of 

 differently coloured glasses. With some of these combinations he 

 felt a sensation of heat, though he had but little light ; while others 

 gave much light, with scarce any degree of heat. Suspecting hence 

 that perhaps certain coloured rays may be more apt to occasion heat, 

 while others, on the contrary, may be more fit for vision, he resolved 

 to put this conjecture to the test of experiments. 



The first set of these experiments relate to the heating power of 

 coloured rays. They were made by admitting successively each dif- 

 ferently coloured ray of a prismatic spectrum, through a proper aper- 

 ture in a pasteboard, on a thermometer whose bulb was blackened, 

 while another similar thermometer, at a certain distance, showed the 

 temperature of the ambient air. The general results here were, that 

 the temperature, or rather the power of heating of the red ray, is 

 greater than any other, bearing a proportion to that of the green ray 

 as 9 to 4, and to the violet, the least calorific, as 13 to 4. 



The next series of experiments was on the illuminating power of 

 coloured rays. These were simply made by viewing through a mi- 

 croscope certain opaque bodies, consisting of minute particles, and 

 illuminated successively by different coloured rays. These sub- 

 gtances were red, green, and black paper, a piece of brass, a nail, and 

 a guinea. 



The uncommon variegated appearances of the metals, and especially 

 of the iron nails, occasioned by their very minute and differently ar- 

 ranged particles, is here mentioned both as an object of admiration, 

 and as singularly conducive to the purposes of the present inquiry ; 

 the greater or smaller number of these particles that became discern- 

 ible by the different coloured rays affording a kind of scale of com- 

 parison which led to the inferences here laid down. 



These general inferences are, that the red-making rays are very 

 far from having the illuminating power in any eminent degree ; that 

 the orange possesses more of it than the red; and the yellow still 

 more ; that the maximum of illumination lies in the brightest yellow 

 or palest green ; that the green itself is nearly equally bright with 



