BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 571 



Herschel, W.: Synopsis oi-- thk Wkitixgs of— Continuod. 



A. D. Vol. P. 



moutatioa, iVirtion. etf.. us well iis linut in its various states of 

 being, latent, spcciiic, absolute, or sensible, may bo explained on 

 tlic princij)le of beat-making rays, and vibrations occasioned by 

 tbem in the parts of bodies ; yet this is not intended at pi'esent to 

 be any part of what I shall endeavor to establish. 

 "I must also remark that in using the word rays I do uot mean to 

 oppose, much less to countenance, the opinion of those philosophers 

 who still believe that light itself comes to us from the sun, not by 

 rays, but by the supposed vibrations of an elastic ether, every- 

 where diffused throughout space. I only claim the same privilege 

 for the rays that occasion heat which they are willing to allow to 

 those that illuminate objects. For, in what manner soever this 

 radiance may be effected, it will be fully proved hereafter that the 

 evidence, either for rays or for vibrations which occasion heat, 

 stands on the same foundation on which the radiance of the illumin- 

 ating principle, light, is built." 

 "We shall begin with the heat of luminous bodies in general, such 

 as, in the first place, we have it directly from the sun ; and as, in 

 the second, we may obtain it from terrestrial flames, such as torches, 

 candles, lamps, blue-lights, etc. 

 1800 90 295 " Our next division comprehends the heat of coloured radiants. This 

 we obtain, in the first place, from the sun, by separating its rays in 

 a prism ; and, in the second, by having recourse to culinary fires, 

 openly exposed. 

 "The third division relates to heat obtained from radiants, where 

 neither light nor colour in the rays can be perceived. This, as I 

 have shown, is to be had, in the first place, directly from the sun, by 

 means of a prism applied to its rays; and, in the second, we may 

 have it from fires inclosed in stoves, and from red-hot iron cooled 

 till it can no longer be seen in the dark. 

 " Besides the arrangement in the order of my experiments which would 

 arise from this division, we have another subject to consider. For, 

 since the chief design of this paper is to give a comparative view 

 of the operations that may be performed on the rays that occasion 

 heat, and of those which we already know to have been eftected on 

 the rays that occasion light, it will be necessary to take a short re- 

 view of the latter. 1 shall merely select such facts as not only are 

 perfectly well known, but especially such as will answer the inten- 

 tion of my comparative view, and arrange them in the following 

 order : 1. Light, both solar and terrestrial, is a sensation occasioned 

 by rays emanating from luminous bodies, which have a power of 

 illuminating objects; and, according to circumstances, of making 

 them appear of various colours. 2. These rays are subject to the 

 laws of reflection. :5. They are also subject to the laws of refrac- 

 29G tion. 4. They are of diflerent relVangibility. 5. They are liable 

 to be stopped, in certain proi)ortious, when transmitted through 

 diaphanous bodies. 0. They are liable to be scattered on rough 

 surfaces. 7. They have hitherto been supposed to have a power of 

 heating bodies; but this remains to be examined. 

 '•The similar propositions relating to heat, which are intended to be 

 proved in this paper, will stand as follows: L Heat, both solar and 

 terrestrial, is a sensation occasioned by rays emanat ing from candent 

 substances, which have a power of heating bodies. 2. These raya 



