BIBLIOGRAPHY OF IIERSCIIKL's WRITINGS. 569 



Herschel, W.: Syxopsis of the Wiutin(;.s of— Contimicd. 

 A. D. Vol. P. 



invisible heat of rcd-Iiot iron, iinnliialiy cool. mI till it ceases lo sliine, 

 has the nionientiiui of the invisible rays which, in tlie solar si)cctrum 

 viewed by daylight, go to the confines of the red ; and this will afford 

 an easy solution of the r<'tleetion of invisible heat by concave mir- 

 rors." 

 1800 90 273 AppUeaiion of the lltsvU of the foir;ioiin/ oh.scrr(i(ion>i to the Method of 

 rirwhig the Sun advantageomlij with Tvlescopex of lartje Apertures and 

 hiijh matjuiftjinci Powers. 



'274 Eelation of experience which led to investigation. 



275-276 Experiments on the absori)tion of various media. 



277 Telescopic Experiments. 



277-282 Description of twenty-seven experiments to determi ne the best form 

 of sun-glass for telescopes. 



283 Certain precautions to be observed in using telescopes on the sun. 

 [Dated] Slough, near Windsor, March 8, 1800. 



1800 90 284 Experiments on the Befranfiibilitif of the invisible liajis of the Sun. By 

 William HKRScnEL, LL. D., F. R. S. Read April 24, 1800. 



284 Description of method. Three sensitive thermometers were placed 



upon a small stand upon which a spectrum of the sunlight was 

 caused to fall. 



285 Report of first experiment. A thermometer J inch beyond limit of 



visible red rose 6| degrees in 10 minutes. 



286 Report of second experiment. As a result it was ''evident that there 



was a refraction of rays coming from the sun, which, though not 

 tit for vision, were yet highly invested with a power of occasioning 

 heat." 



287-288 Experiments at the violet end of the spectrum. 



"From these last experiments I was now sufficiently persuaded that 

 no rays which might fall beyond the violet could liave any percepti- 

 ble power, either of illuminating or of heating ; and that both these 

 jjowers continued together throughout the prismatic spectrum, and 

 ended where the faintest violet vanishes." 



289-290 Experiments to determine the situation of the maximum of the 

 heating powei'. 



291 "The 5th and 6th experiments show, that the power of heating is ex- 

 tended to the utmost limits of the visible violet rays, but not beyond 

 them, and that it is gradually impaired as the rays grow more refran- 

 gible. 

 "The four last experiments prove, that the maximum of the heat- 

 ing power is vested among the invisible rays; and is probably not 

 less than half an inch lieyond the last visible ones, when projected 

 in the manner before mentioned. The same experiments also show, 

 that the sun's invisible rays, in their less refrangible state, and con- 

 siderably beyond the maximum, still exert a heating power fully 

 equal to that of red-coloured light ; and that, consequently, if we may 

 infer the quantity of the efficient from the effect produced, the invis- 

 ible rays of the sun probably far exceed thcA-isible ones in number. 

 "To conclude, if we call light, those rays which illuminate objects, and 

 radiant heat, those which heat bodies, it may be inquired whether 

 light be essentially different from radiant heat ? In answer to which 

 I would suggest, that we are not allowed, by the rules of philoso- 

 phizing, to admit two different causes to explain certain effects, if 



