BIBLIOGRAPHY OF IIEKSCHEL's WRITINGS. 569 



Herschel, W.: Syxopsis ov tui: \\'i;ni.\(;s OF— Colli i lined. 

 A. D. Vol. P. 



iiivisilil(']ic;if()fic(l-li()t iron, or;i,iii;,lly cooled till it ceases 1o. shine 

 has the moment nm oflhci invisilile rays wliich, in the solar spectrum 

 viewed by dayli<;lit, go to the confines of the red; and this will afford 

 an easy solution of Ihe nlleciion of Jnvisil>le heat by concave mir- 

 rors." 

 1800 90 27:5 AppVifaiion of ihe, llvsvU of the forp<joinfi ohsenmlionx to the Method of 

 viewing the Sun advantdf/eoitsly with Telescopes of larrjc Apertures and 

 high magnifjjing Powers. 



274 Relation of experience which led to investigation. 



275-27G Experiments on the absorption of various media. 



277 Telescopic Experiments. 



277-282 Description of twenty-seven exi)eriments todetermine the best form 

 of sun-glass for telescopes. 



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

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



1800 90 284 Experiments on the Eefrangihility of the invisible Rays of the Sun. By 

 William Herschel, 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 A inch beyond limit of 



visible red rose 0^ 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 Hun, which, thougli not 

 fit 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 sufTiciently persuaded that 

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

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

 powers continued together throughout the prismatic spectrum, and 

 ended where the faintest violet vanishes:" 



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

 heating power. 



291 "The 5th and Gth experiments show, that tli(> 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 beyond 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 refrangilile 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 etiect produced, the invis- 

 ible rays of the sun probably far exceed the visible 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 dill'erent from radiant heat ? In answer to wliich 

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

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



