gap On the healing Power of coloured Rays. 



VI. 



Invejligation of the Powers of the Prifmatic Colours to heat and illuminate ObjeBs ; with 

 Remarks that prove the different RefrangibUity of radiant Heat. To which are added, 

 an Inquiry into the Method of viewing the Sun advantageoufly, with Telefcopes of large 

 Apertures and high magnifying Powers, and Experiments on the Refrangibility of the 

 invijible Rays of the Sun. By William Herschel, L.L.D. F.R.S*. 



HIS eminent philofopher begins his firft paper by obferving, that it is fometimes of 

 great ufe in natural philofophy, to doubt of things which are commonly taken for granted ; 

 efpecially as the means of refolving any doubt when once it is entertained, are often within 

 our reach. Whence it may be affirmed, that any experiment which leads us to inveftigate 

 what was before admitted upon truft, may become of great utility to natural knowledge. 

 Thus for jnftance, when we fee the effe£t of the condenfation of the fun's rays in the focus 

 of a burning lens, it feems to be natural to fuppofc, that every one of the united rays contri- 

 butes its proportional fliare to the intenfity of the heat which is produced; and we fhould 

 probably think it highly abfurd, if it were aflerted, that many of them had but little con- 

 cern in the combuftion or vitrification which follows when an objedl: is put into that focus. 

 From thefe confiderations, he has thought fit to mention what led him to furmife, that the 

 power of heating and illuminating obje£ls might not be equally diftributed among the va- 

 rioufly coloured rays. 



In a variety of experiments occafionally made, relating to the method of viewing the fun 

 with large telefcopes to the beft advantage, he ufed various combinations of differently co- 

 loured darkening glafles. What appeared remarkable was, that when he ufed fome of 

 thera he felt a fenfation of heat, though he had but little light; while others gave much 

 light, with fcarcely any fenfation of heat. Now as in thefe different combinations the fun's 

 image was alfo differently coloured, it occurred to him that the prifmatic rays might have 

 the power of heating bodies very unequally diftributed among them ; and as he judged it 

 right in this refpe£l to entertain a doubt, it appeared equally propw to admit the fame with 

 regard to light. If certain colours ihould be more apt to occafion heat, others might, on 

 the contrary, be more fit for vifion by pofTeffing a fupetior illuminating power; and at all 

 events, it would be proper to recur to experiments for a decifion. 



In the firft fcries of experiments on the heating po'wer of the coloured rays of theftm, a piece 

 of pafteboard was mounted in a frame, fo that its obliquity could be varied with refpefl to 

 the horizon, nearly in the fame manner as in the common table looking glafs. Through 

 this pafteboard was cut a notch, or flit, of width a little larger than the ball of a thermo- 



• Abridged from two papers in the Philofophical Tranfaftions for 1800, pages 155-— 292. The titles 

 nf both arc united in the above, with no other variation than the infeition of the conjunftion and before 

 t!)e word experiments, which begins the fecond title ; and the necefTary change of tlie word ;.; for are.—-'^. 



meter. 



