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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 advantageously, with 

 Telescopes of large Apertures and high magnifying Powers. 

 By William Herschel, LL. D. F. R. S. 



Read March 27, 1800. 



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 experi- 

 ment which leads us to investigate the truth of what was before 

 admitted upon 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 which is 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 mention what gave rise to a surmise, that the power of 

 heating and illuminating objects, might not be equally distri- 

 buted among the variously coloured rays. 



In a variety of experiments I have occasionally made, relating 

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