fo as to make the coloured image of the fun ftationary upon the table where the microscope was placed, I caufed the differently-coloured rays to fall fucceffively on the object, by advancing the microfcope into their light. The magnifying power was 27 times. "In changing the illumination, by admitting different colour, it always becomes neceffary to readjuft the inftrument. It is well known, that the different refrangibility of the rays will fenfibly affect the focal length of object-glaffes; but in compound vifion, fuch as in a microscope, where a very fmall lens is made to caft a lengthened fecondary focus, this difference becomes ftill more confiderable. "By an attentive and repeated infpection, I found that my object was very well feen in red, better in orange, and still better in yellow; full as well in green, but to lefs advantage in blue; indifferently well in indigo, and with more imperfection in violet." Several other fubftances, fuch as red paper, green paper, a piece of brafs, &c. were likewife viewed in differently coloured lights, and their appearances are particularly defcribed. "From thefe obfervations, which agree uncommonly well, with refpect to the illuminating power affigned to each colour, we may conclude, that the red-making rays are very far from having it in any eminent degree. The orange poffefs more of it than the red; and the yellow rays illuminate objects ftill more perfectly. The maximum of illumination lies in the brightest yellow, or paleft green. The green itself is nearly equally bright with the yellow; but, from the fulf deep green, the illuminating power decreases very fenfibly. That of the blue is nearly upon a par with that of the red; the indigo has much less than the blue; and the violet is very deficient. "With regard to the principle of diftin&inefs, there appears to be no deficiency in any one of the colours, &c." Radiant Heat is of different Refrangibility. "I must now remark that my foregoing experiments afcertain be yond a doubt, that radiant heat, as well as light, whether they be the fame or different agents, is not only refrangible, but is alfo fubject to the laws of the difperfion arifing from its different refrangibility; and,` as this fubject is new, I may be permitted to dwell a few moments upon it. The prifm refracts radiant heat, fo as to feparate that which is lefs efficacious, from that which is more fo. The whole quantity of radiant heat contained in a fun-beam, if this different refrangibility did not exift, muft inevitably fall uniformly on a fpace equal to the area of the prifm; and, if radiant heat were not refrangible at all, it would fall upon an equal space, in the place where the fhadow of the prifm, when covered, may be feen. But, neither of thefe events taking place, it is evident that radiant heat is fubject to the laws of refrac tion, and alfo to thofe of the different refrangibility of light. May not this lead us to furmife, that radiant heat confifts of particles of light of a certain range of momenta, and which range may extend a little farther, on each fide of refrangibility, than that of light? We have fhewn, that in a gradual expofure of the thermometer to the rays L 2 of of the prifmatic fpectrum, beginning from the violet, we come to the maximum of light, long before we come to that of heat, which lies at the other extreme. By feveral experiments, which time will not allow me now to report, it appears that the maximum of illumination has little more than half the heat of the full red rays; and from other experiments, I likewife conclude, that the full red falls ftill fhort of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this cafe, radiant heat will at leaft partly, if not chiefly, confift, if I may be permitted the expreffion, of invifible light; that is to fay, of rays coming from the fun, that have fuch a momentuin as to be unfit for vifion. And, admitting, as is highly probable, that the organs of fight are only adapted to receive impreffions from particles of a certain momentum, it explains why the maximum of illumination fhould be in the middle of the refrangible rays; as thofe which have greater or lefs momenta, are likely to become equally unfit for impreffions of fight. Whereas, in radiant heat, there may be no fuch limitation to the momentum of its particles. From the powerful effects of a burning lens, however, we gather the information, that the momentum of the terreftrial radiant is not likely to exceed that of the fun; and that, confequently, the refrangibility of calorific rays cannot extend much beyond that of colourific light. Hence we may alfo infer, that the invifible heat of red-hot iron, gradually cooled till it ceases to fhine, has the momentum of the invifible rays which, in the folar fpectrum viewed by day-light, go to the confines of red; and this will afford an eafy folution of the reflection of invisible heat by concave mirrors." Application of the Refult of the foregoing Obfervations, to the Method of viewing the Sun advantageously, with Telescopes of large Apertures and magnifying Powers. 1 The particulars of this fection, and of the next, under the title of Telescopic Experiments, do not admit of being fufficiently abridged to be reported here. XIV. Experiments on the Refrangibility of the invifible Rays of the Sun. By William Herfchel, LL. D. F. R. S. In the fection of the preceding paper, on radiant heat, which we have transcribed in full, the ftrong probability is afferted, of the range of radiant heat being more extenfive than that of the coloured fpectrum. The experiments which are related in the prefent paper place the matter beyond all doubt, and furnish, at the fame time, the explanation of feveral curious phænomena. The apparatus, with which thofe experiments were performed, is delineated in a plate, without which it would not be eafy to give a diftinct defcription of them. It may fuffice to fay, that Dr. Herfchel placed fome thermometers in different parts of the prifmatic fpectrum, as alfo in places adjoining to it; and by a long feries of obfervations, in I which this very able philofopher ufed every neceffary precantion, he was at length enabled to form the following deduc tions: "The first four experiments prove, that there are rays coming from t fun which are lefs refrangible than any of those that affect the fight.. They are invested with a high power of heating bodies, but with none of illuminating objects; and this explains the reafon why they have hitherto efcaped unnoticed. My prefent intention is, not to affign the angle of the leaft refrangibility, belonging to thefe rays, for which purpofe more accurate, repeated, and extended experiments are required. But, at the distance of 52 inches from the prifin, there was still a confiderable heating power exerted by our invifible rays, one inch and a half beyond the red ones, measured upon their projection on a horizontal plane. I have no doubt but that their efficacy may be traced ftill fomewhat farther. "The fifth and fixth experiments fhew, that the power of heating is extended to the utmoft limits of the vifible violet rays, but not beyond them; and that it is gradually impaired, as the rays grow more refrangible. "The four laft experiments prove, that the maximum of the heating power is vefted among the invifible rays; and is probably not lefs than half an inch beyond the latt visible ones, when projected in the manner before-mentioned. The fame experiments alfo fhew, that the fun's invifible rays, in their lefs refrangible ftate, and confiderably beyond the maximum, ftill exert a heating power fully equal to that of red coloured light; and that, confequently, if we may infer the quantity of the efficient from the effect produced, the invifible rays of the fun probably far exceed the vifible ones in number. To conclude, if we call light, thofe rays which illuminate objects, and radiant heat, thofe which heat bodies, it may be inquired, whether light be effentially different from radiant heat? In answer to which I would fuggeft, that we are not allowed, by the rules of philofophizing, to admit two different caufes to explain certain effects, if they may be accounted for by one. A beam of radiant heat, emanating from the fun, confifts 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 refracted, and have the leaft efficacy. We have traced thefe calorific rays throughout the whole extent of the prifmatic fpectrum; and found their power increafing, while their refrangibility was leffened, as far as to the confines of red-coloured light. But their diminishing refrangibility, and increafing power, did not stop here; for we have purfued them a confiderable way beyond the prifmatic Spectrum, into an invisible ftate, ftill exerting their increafing energy, with a decrease of sefrangibility up to the maximum of their power; and havé alfo traced them to that state where, though ftill lefs refracted, the r energy, on account, we may fuppofe, of their now failing denfity, decreased pretty fast; after which, the invifible thermometrical Spectrum, if I may to call it, foon vanished. It this be a true account of folar heat, for the fupport of which I appeal to my experiments, it remains only for us to admit, that fuch of of the rays of the fun as have the refrangibility of thofe which are contained in the prifmatic fpectrum, by the conftruction of the organs of fight, are admitted, under the appearance of light and colours; and that the reft, being stopped in the coats and humours of the eye,' act upon them, as they are known to do upon all other parts of our body, by occafioning a fenfation of heat.” XV. Experiments on the folar, and on the terreftrial Rays that occafion Heat; with a comparative View of the Laws to which Light and Heat, or rather the Rays which occafion them,, are fubject, in Order to determine whether they are the fame, or different. By William Herfchel, LL. D. F. R. S. This third, and elaborate paper, by the fame author, may be confidered as a continuation of the preceding fubject. It contains a confiderable number of experiments, it determines feveral remarkable particulars, and the whole is intermixed with proper and inftructive reafoning. Dr. Herfchel begins, by declaring the meaning of what he calls the rays that occafion beat; namely, that by this he does not mean to adopt, or to allude to any theory, either of the emanation of a calorific fluid, or of the communication of motion to a fluid, which is difperfed throughout space, and which may be caufed to produce heat by a particular agitation, &c. He then diftinguishes heat into fix different kinds; namely, three folar, and three terreftrial; but as the latter trialy refemble the former, he reduces the number to three; namely, 1. that which we receive directly from the fun, or may obtain by the flame of torches, lamps, candles, &c. 2. The heat of coloured radiants, which is obtained by feparating the rays of the fun with a prifm, or by having recourfe to culinary fires. 3. The heat of radiants, which yield neither light nor colours. Since the object of this paper is to give a comparative view of the operations that may be performed on the rays that occafion heat, and of those which can be effected on those that occafion ligh', Dr. H. has thought proper to make a recapitulation of feveral facts which relate to the latter. This ftatement is followed by the account of twenty experiments, which were performed with different inftruments, and the apparatus is accurately delineated on five plates. With refpect to the refult of thofe experiments, of which we fhall fubjoin the titles, it is only in our power to fay in general that they prove, beyond a doubt, that the rays which occafion heat, both folar and terrestrial, are in every state subject to the laws of reflection, and to the laws of refraction. Exp. 1. Reflection of the heat of the fun. 3. Reflection of the heat that accompanies the folar prifmatic colours. 4. Reflection of the heat of a red-hot poker. 5. Reflection of the heat of a coal fire, by a plain mirror. 6. Reflection of fire-heat, by a prifm. 7. Reflection of invifible folar heat. Reflection and condenfation of the invifible folar rays. 9. Reflection of invifible culinary heat. II. 12. Reflection of the invifible rays of heat of a poker, cooled from being red-hot till it could no longer be seen in a dark place. Refraction of folar heat. Refraction of the heat of a candle. 13. Refraction of the heat that accompanies the co- 14. Refraction of the heat of a chimney-fire. 17. Refraction of the invifible rays of folar heat. 19. Refraction of invisible culinary heat. 20. Confirmation of the 19th experiment. XVI. Chemical Experiments on Zoophytes; with fome Obfervations on the component Parts of Membrane. By Charles Hatchett, Efq. F. R.S. Mr. Hatchett has beftowed much labour on the analysis of fhell and bone, as appears from his papers which have been publifhed in the Phil. Tranf. previous to the prefent volume. The prefent rather long and elaborate paper is a continuation of the fame fubject. It is divided into three parts; namely, ift. Experiments on Zoophytes; 2dly. Obfervations on the foregoing Experiments; and, 3dly. Obfervations on the component Parts of Membrane. To all which is prefixed, a fhort Statement of the principal Facts, which had already been established relatively to the nature of fhells, cruftaceous fubftances, and bones. The experiments of the first part were performed on the following fubftances, which were treated with acids and other |