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diagrams and preparations of the experiments are delineated in à plate which follows the paper). But it being not in our power to give a very concise, and at the same time a sufliciently clear idea of those particulars, we mall conclude with an enumeration of the particulars that are explained by them; viz.

ro it. Why air heated by the moderate warmth of the sun's rays, occasions objects to appear doubled and inverted,

vs 2ndly. Why retraction, by a higher degree of heat, gives an additional image, which is not inverted.

« zdly. In what state of evaporation the increase of the air's dena fity brings distant objects into view by unusual elevation.

H4thly. Under what circumstances evaporation may also produce an inverted image less elevated.

* And it is probable, "that the same reasoning will afford a ready explanation to other varieties of terreitrial refraction that may have been, or may hereafter be observed.”

XIII. Investigation of the Powers of the prismatic Colours to heat, and illuminate Objects ; with Remarks, that prove the dif. ferent Refrangibility of radiant Heat. To which is added, an Inquiry into the Method of viewing the Sun advantageously, with Telescopes of large ri pertures, and high magnifying Powers. By William Herschel, LL. D. F.R.S. . The various experiments which this indefatigable astronomer has made, at different times, for the purpose of observing the sun through telescopes, in a manner that might be less hurtful to ine eye, and the use of differently coloured darkening glasses, had truck him at different times with a remarkable peculiarity of effects ; for with some of those darkening glasses

he felt a considerable sensation of heat, whilst they transmitted · but little light; whereas others transmitted much light, and.

scarce any sensation of heat. iNow," lays he, “ as in these different combinations the sun's '. image was also differently coloured, it occurred to me, that the pris. matic rays might have the power of hearing bodies very unequally distributed among them; and, as I judged it right in this respect to entercain a doubl, it appeared equally proper to admit the same with regard to light. If certain colours should be more apt to occasion heat, others might, on the contrary, be more fit for vifion, by potrifling a superior illuminating power."

"The well-imagined, and well-executed experiments, which · Dr. H. made in consequence of this supposition, verified his

hypothesis, and opened a vast field of speculation and experimental enquiry to the eyes of philosophers. The account of EL

the Drit. crit. VOL. XVII, feb. 1801.

the experiments and observations which forms this most in. teresting paper, is arranged under diftinct heads, of which we shall endeavour to give a concise idea.

Experiments on the heating Power of coloured Rays. The general method of conducting such experiments is dee scribed by this author in the following manner :

6 I fixed,” says he, “ a piece of pasteboard in a frame mounted upon a stand, and moveable upon two centres. In the pasteboard I cut an opening, a little larger than the ball of a thermoineter, and of a sufficient length to let the whole extent of one of the prismatic colours pass through. I then placed three thermometers upon small inclined planes : their balls were blackened with japan ink. Their balls being very small, made them of exquisite sensibility. The scales of all were properly disengaged from the balls.

" I now placed the stand, with the framed pasteboard and the therm mometers, upon a small plain board; that I might be at liberry to move the whole apparatus together, without deranging the relative situation

of the different parts. i ." This being done, I set a prism, moveable on its axis, into the

upper part of an open window, at right angles to the folar rays; and turned it about till its refracted coloured spectrum became stationary, apon a table placed at a proper distance from the window.

The board containing the apparatus was now put on the table, and set in such a manner as to let the rays of one colour pass through the opening in the pasteboard. The moveable frame was then adjusted to be perpendicular to the rays coming from the prism; and the inclined planes carrying the three thermometers, with their balls arranged in a line, were set to near the opening, that any one of them might easily be advanced far enough to receive the irradiation of the colour which passed through the opening, while the rest remained close by, under the shade of the pasteboard."

This description is accompanied with a delineation of the apparatus.

The conclusion drawn from the result of the experiments which were performed with this apparatus, and which are particularly stated in the paper, is, that the heating power of red rays is to the heating power of green rays, as 550 to 242; and that the hearing power of red rays is to that of violet rays, as 35 10 10.

Experiments on the illuminating Power of coloured Rays.

The experiments which are related under this head, had two objects in view, namely, to ascertain the power of illuminating

which belongs to each particular coloured say, and their aptness .. for giving distinct vision.

« I placed,” says Ds. H. “ an object that had very minute parts, under a double microscope; and, having set a prism in the window,


so as to make the coloured image of the sun stationary upon the table where the microscope was placed, I caused the differently-coloured rays to fall successively on the object, by advancing the microscope into their light. The magnifying power was 27 times.

• In changing the illumination, by admitting a different colour, it always becomes necessary to readjust the instrument. It is well known, that the different refrangibility of the rays will sensibly affect the focal length of object-glasses; but in compound vision, such as in a microscope, where a very small lens is made to cast a lengthened fecondary focus, this difference becomes still more considerable.

“ By an attentive and repeated inspection, I found that my object was very well seen in red, better in orange, and still better in yellow ; full as well in green, but to less advantage in blue; indifferently well in indigo, and with more imperfection iu violet.” · Several other substances, such as red paper, green paper, a piece of brass, &c. were likewise viewed in differently coloured lights, and their appearances are particularly described.

“ From these observations, which agree uncommonly well, with respect to the illuminating power assigned to each colour, we may conclude, that the red-making rays are very far from having it in any emi. nent degree. The orange postess more of it than the red; and the yellow says illuminate objects still more perfely. The maximum of illumination lies in the brightest yellow, or palest green. The green itself is nearly equally bright with the yellow ; but, from the full deep green, the illuminating power decreases very sensibly. 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 distinctness, there appears to be no deficiency in any one of the colours, &c.”

Radiant Heat is of different Refrangibility. so I must now remark that my foregoing experiments ascertain les yond a doubt, that radiant hear, as well as light, whether they be the fame or different agents, is not only refrangible, but is also subject to the laws of the dispersion arising from its different refrangibility ; and, as this subject is new, I may be permitted to dwell a few moments upon it. The prisma refracts radiant hear, fo as to separate that which is less efficacious, from that wbich is more fo. The whole quantity of radiant heat contained in a fun-beam, if this different refrangibility did not exist, muft inevitably fall uniformly on a space equal to the area of the prism; and, it radiant heat were not refrangible at all, it would fall upon an equal space, in the place where the shadow of the prism, when covered, may be seen. But, neither of these events taking place, it is evident that radiant heat is subject to the laws of refrace tion, and also to those of che different refrangibility of light. May not this lead us to surmise, that radiant heat consists of particles of light of a certain range of momenta, and which range may extend a lircle fartber, on each side of refrangibility, than that of light? We have thewn, that in a gradual exposure of the thermometer to the rays Le 2



of the prismatic spectrum, 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 several experiments, which iime will not allow me now to report, it appears that the maximum of illumination has limle more than half the heat of the full red rays; and from other experiments, I likewise conclude, that the full red falls still fhort of the maximum of heat; which perhaps lies even a little beyond visible refraction. In this case, radiant heat will at least partly, if not chiefly, confitt, if I may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentuin as to be unfit for vision. And, admitting, as is highly probable, that the organs of light are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumi. nation should be in the middle of the refrangible rays; as those which have greater or less momenta, are likely to become equally unfit for impressions of fight. Whereas, in radiant heat, there may be no such limitation to the momentum of its particles. From the powerful effects of a burning lens, however, we gather the information, that the mo. mentum of the terrestrial radiant is not likely to exceed that of the fun; and that, consequently, the refrangibility of calorific rays cannot extend much beyond that of colourific light. Hence we may also infer, thai the invisible heat of red-hot iron, gradually cooled till it ceases to thine, has the mornenium of the invisible rays which, in the salar fpecirum viewed by day-light, go to the confines of red ; and this will afford an easy solution of the reflection of invisible heat by concave mirrors." Application of the Result of the foregoing Observasions, to the

Method of viewing the Sun advantageously, with Telescopes of large Apertures and magnifying Powers.

The particulars of this section, and of the next, under the title of Telescopic Experiments, do not admit of being sufficiently abridged to be reported here.

XIV. Experiments on the Refrangibility of the invisible Rays of the Sun. “By William Herschel, LL. D. F.R.S.

In the section of the preceding paper, on radiant heat, whicla we have vanscribed in tull, the throng probability is afseried, of the large of radiant heat being more extensive than that of the coloured specii'um. The experimenis which are related in the present paper place ihe matter beyond all doubt, and furnish, at the same time, the explanation of several curious phænomena.

The apparatus, with which those experiments were performed, is delineated in a plate, without which it would not be easy to give a distinct description of them. It may suffice to say, that Dr. Herschel placed some thermometers in different paris of the prismatic spectrum, as also in places adjoining to it; and by a long series of observations, in

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which this very able philosopher used every necessary precantion, he was at lengih enabled to form the following deductions :

« The first four experiments prove, that there are rays coming from , e fun which are less 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 reason why they have hitherto escaped unnoticed.

"My present intention is, not to aflign the angle of the least re. frangibility, belonging to thefe rays, for which purpose more accurate, repeated, and extended experiments are required. But, at the distance of 52 inches from the prison, there was still a considerable hearing power exerted by our invisible 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 still somewhat farther, , ,

" The fifth and fixth experiments Thew, that the power of heating is extended to the utmost limits of the visible violet rays, but not beyond them; and that it is gradually impaired, as the rays grow more refrangible.

• The four last experiments prove, that the maximum of the heating power is vefted among the invisible rays; and is probably not less than half an inch beyond the last visible ones, when projected in the manner before-mentioned. The same experiments also new, that the fun's invisible rays, in their less refrangible state, and considerably beyond the maximum, ftill exert a hearing power fully equal to that of red coloured light; and that, consequently, if we may infer the quantity of the efficient from the effect produced, the invisible rays of the fun' probably far exceed the vifible ones in number.

or To conclude, if we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, ii may be inquired, whether light be essentially different from radiant heat?' In answer' to which I would suggest, that we are not allowed, by the rules of philosophizing, to admit two different causes to explain certain effects, if they may be accounted for by one. A beam of radiant heat, emanating from the sun, consists of rays that are differently refrangible. The range of their extent, when dispersed by a prism, begins at violet coloured light, where they are moft refracted, and have the least efficacy. We have traced these calorific rays throughout the whole extent of the prismatic spectrum ; and found their power increasing, while their refrangibility was lefsened, as far as to the confines of red-coloured light. But their diminishing refrangibility, and increasing power, did not stop here; for we have pursued them a considerable way beyond the prismatic Spectrum, into an invisible flate, till exerting their increasing energy, with a decrease of sefrangibiliiy up to the maximum of their power; and have also traced them to that itare where, though still less refract: ed, the r energy, on account, we may suppose, of their now failing density, decreased pretiy fait; after which, the invisible thermometrical Spearum, if I may lo call it, foon vanished. **«It this be a true account of solar heat, for the support of whichi I appeal to my experiments, it remains only for us to admir, that fuch


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