RAYS. 



of experiments and his reafoning upon th;m, is in the follow- 

 ing manner : " if," fays he, " we call light thole rays 

 which illuminate objefts, and radiant heal, thofe which 

 heat bodies, it may be inquired whether liglit be eflentially 

 different from radiant heat ? In anfwer to which I would 

 fuggeft, that we are not allowed by the rules of philofo- 

 phizing to admit two different caufes to explain certain 

 effefts, if they may be accounted for by one. A beam of 

 radiant heat emanating from the fun, confiifs of rays that 

 are differently refrangible. The range of their extent, 

 when difperfed by a prifm, begins at violet-coloured light, 

 where they are moft refrafted, and have the leafl efficacy. 

 We have traced thefe calorific rays throughout the whole ex- 

 tent of the prifmatic fpeftrum, and found their power in- 

 creafing, while their refrangibility was lefTi-ned, as far as 

 to the confines of red-coloured light. But their dimi- 

 nifhing refrangibility, and increaflng power, did not Hop 

 here : for we have purfued them a confiderable way b.fyond 

 ihs prifmatic fpeSrum into an' invifible Hate, Hill exerting 

 their increafii)g energy, with a decreafe of refrangibility 

 up to the maximum of their power ; and liave alfo traced 

 them to that ftate, where, though ftill lefs refrafted, their 

 energy, on account, we may fuppofe, of their now failing 

 denfity, decreafed pretty fall ; after which the invifible ther- 

 mometrical fpearum, if I may fo call it, foon vanilhed." — " It 

 this," continues our author, " be a true account of fokir 

 heat, for the fupport of which I appeal to ray experiments, 

 it remains only for us to admit, that fuch of the rays of 

 the fun as have the refrangibility of thofe which are con- 

 tained in the prifmatic fpeclrum, by the conllruftion of the 

 organs of fight, are admitttd, under the appearance of 

 light and colour ; and that the reft, being llopped in the 

 coats and humours of the eye, aft upon them, as they are 

 known to do upon all other parts of our body, by occafion- 

 ing a fenfation of heat." 



Although, as we have above ftated, the rays of light 

 and thofe of heat poifefs many fimilar properties, yet 

 Dr. Herfchcl has fhewn, that there are fome ftriking and 

 lubftantial differences between them. The rays of heat are 

 of a much more extenfive refrangibility than thofe of light, 

 as our author has clearly and incontcllibly demonllrated 

 both by reafoning and experiment : nor do thofe rays agree 

 either in their mean refrangibility, nor in the fituation of 

 their maxima. Where we have mofl light, there is but little 

 heat, and where we have moil heat, we find no light at all. 

 (See Refrangibility.) It is found, that the fines of refrac- 

 tion of the heat-making rays are in a conftant ratio to the fines 

 of incidaiice; but that the focus of the rays of heat in burning 

 glalles is different from the focus of tlie rays of light : that of 

 heat being farther removed from the lens than the focus of 

 light, probably not lefs than a quarter of an inch : the heat at 

 lialf an inch beyond the focus of light being ftill equal to 

 that in the focus. Although fight and heat are both re- 

 frangible, the ratio of the lines of incidence and refraftion 

 of ttie mean rays is not the fame in both. Kent is evidently 

 lefs refrangible than light. From experiments relating to 

 the tranfmillion of light and heat through diaphanous bodies 

 it appears, that no kind of regularity takes place in the pro- 

 portion of rays of one fort and of another, which are ftopped 

 ill their paflage. Heat and light feem to be entirely uncon- 

 necled, and the rays that occalion them are different. We 

 have feveral tables, formed from the author's experiments, 

 and (hewing the quantities of light and heat intercepted by 

 different fubllanccs. The blueilli-white and flint-glades, 

 c. gr. ftop nearly three times as much heat as light, whereas 

 ttie greenifh crown glafs flops only about one-fourth more 

 of tlit' former than of the latter. 



From a table exhibiting the effefts of coloured glafles, it 

 appears, that a yellow glafs flops only 333 rays of heat, but 

 (lops 819 of ligtit ; on the contrary, a pale blue (lops 

 bi2 rays of heat, and but 684 of light. Again, a dark 

 blue glafs flops only 362 rays of heat, but intercepts 801 

 of liglit ; and a dark red glafs ftops no more than 606 rays 

 of heat, and yet intercepts nearly all the light, Icarcely one 

 ray out of 5000 being able to make its way through it. For 

 the conclulions deduced from tfiefe tables, as they evince 

 the non-identity of the rays of heat and thofe of light, and 

 for the tables themfelves, we mull refer to the author's own 

 paper, ubi infra. We are reflrained by oar limits from pur- 

 fuing this curious fubjeft, and from entering on a detail of 

 Dr. Herfchel's experiments, illuflrating and confirming prin- 

 ciples, tlie difcovery of which mull be allowed to be one of 

 the greatell that has been made fince the days of Newton, 

 although ttie theories of fome fpeculative philofophers might 

 have led to it a few years earlier. Dr. Herfchel was occu- 

 pied in determining the properties of various kinds of co- 

 loured glafs, which rendered th«m more or lefs fit for 

 enabling tlie eye to view the fun through a telefcope ; and 

 for this purpoie it was neceflary to inquire which of the 

 rays would furnifh the greatell quantity of light, without- 

 fubjeftlng the eye to the inconvenience of unnecefi'ary heat. 

 He firll obferved that the heat became more and more con- 

 fiderable as the thermometer approached the extreme red 

 rays in the prifmatic fpeftrum ; and purfuing the experi- 

 ment, he found not only that the heat continued beyond 

 the vifible fpeftrum, but that it was even more intenfe 

 where the thermometer was at a little diftance without the 

 limits of the fpeftrum, than in any point within it ; as we 

 have already ftated. For Dr. Herfchel's communications to 

 the Royal Society on this fubjecl, we refer to the tjoth vo- 

 lume of the Philofophical Tranfaftions for the year 1800, 

 part ii. p. 255, &c. p. 284, &c. p. 293, &c. part iii. 

 p. 437, &c. 



Sir Henry Englefield has repeated Dr. Herfchel's ex- 

 periments with many precautions, and Mr. (fir H.) Davy 

 was a witnefs of their perfeft accuracy. The excefs of heat 

 beyond the fpeClrum was even confiderable enough to be 

 afcertained by the (enfe of warmth occafioned by fhewing it 

 on the hand. 



It was firll obferved in Germany by Ritter, and foon af- 

 terwards in England by Dr. Wollafton, that the muriate of 

 filver is blackened by invifible rays, which extend beyond 

 the prifmatic fpeclrum, on the violet fide. It is therefore 

 probable, that tiiefe black or invifible rays, the violet, blue, 

 green, perhaps the vellow, «nd the red rays of light, and 

 the rays of invifible heat, confti-tute feven different degrees 

 of the fame fcale, diftinguifhed from each other into this 

 limited number, not by natural divifions, but by their effefts 

 on our fenfes ; and we may alfo conclude that there is fome 

 fimilar relation between heated and luminous bodies of dif- 

 ferent kinds. See Young's Phil. Left. vol. i. p. 639. See 

 Heat and Refrac tion. 



M. Delaroche has found, that the rays of invifible heat 

 traverfe glafs with difficulty at a temperature below that of 

 boiling water ; but that they traverfe it with a facility, al- 

 ways increafing with the temperature, as it approaches the 

 point when bodies become luminous, and from thefe experi- 

 ments it would appear, that the modification, whatever it 

 be, which muft be imprefled upon the invifible rays to 

 render them more and more capable of paffing through glafs, 

 makes them approach more and more to the (late in which 

 they muft be when they penetrate our eyes, and occafion the 

 fenfation of vifion. Tlie fame ingenious philofopher has 

 likewife found, that the rays of heat which have palled 



throuofh 



