Part 11 (2/2)
Pairs of stars which were _exactly equal_, were selected bythe aperture of one telescope directed to a bright star, and keeping the other telescope unchanged and directed to a fainter star, the two stars could be equalized in light, and, froht of this pair of stars could be accurately computed, and so on for other pairs This was the first use of the eneral results were that the stars of the first nitude would still remain visible to the naked eye, even if they were at a distance from us _twelve_ times their actual distance
This method received a still further developained all the infor He prepared a set of telescopes collecting 4, 9, 16, etc
(2 2, 3 3, 4 4, etc), tiht as the naked eye
These were to extend the determinations of distance to the telescopic stars For example, a certain portion of the heavens which he examined contained no star visible to the naked eye, but many telescopic stars
We cannot say that no one of these is as bright in itself as sonitude stars The se number of stars; these e, as the stars just visible to the naked eye But first-nitude stars, like _Sirius_, _Procyon_, _Arcturus_, etc, become just visible to the eye if removed to twelve times their present distance Hence the stars seen in this first telescope of the set were between twelve and twenty-four times as far from us as _Arcturus_, for example
”At least,” as HERSCHEL says, ”we are certain that if stars of the size and lustre of _Sirius_, _Arcturus_, etc, were removed into the profundity of space I have mentioned, they would then appear like the stars which I saw” With the next telescope, which collected nine tiht into view objects three times more distant, other and new stars appeared, which were then (3 12) thirty-six times farther from us than _Arcturus_ In the same way, the seven-foot reflector showed stars 204 times, the ten-foot 344 tie first-ht froht from the faintest stars seen by the twenty-foot would require 2,700 years (3 900)
But HERSCHEL was now (1817) convinced that the twenty-foot telescope could not penetrate to the boundaries of the Milky Way; the faintest stars of the Galaxy must then be farther from us even than nine hundred tiht must be at least 3,000 years old when it reaches us
There is no escaping a certain part of the consequences established by HERSCHEL It is indeed true that unless a particular star is of the sa does not apply to it; in just so far as the average star is less bright than the average brightness of our largest stars, will the numbers which HERSCHEL obtained be diminished But for every star of which his hypothesis is true, we may assert that his conclusions are true, and no one can deny, with any show of reason, that, on the whole, his suppositions must be valid On the whole, the stars which we call faint are farther frohter ones; and, on the whole, the brilliancy of our brightest and nearest stars is not very far froe star in space We cannot yet define the word _very_ by a numerical ratio
The _method_ struck out by HERSCHEL was correct; it is for his successors to look for the special cases and limitations, to answer the question, At a certain distance from us, what are the variations which actually take place in the brilliancy and the sizes of stars? The answer to this question is to be found in the study of the clusters of regular forms, where we _know_ the stars to be all at the saht and Heat, Etc_
Frequently in the course of his astronomical work, HERSCHEL found himself confronted by questions of physics which could not be immediately answered in the state of the science at that ti the dimensions of the stellar universe, he was finally led, as has been shown, to regard the brightness of a star as, in general, the best attainable measure of its distance from us His work, however, was done with telescopes of various dimensions and powers, and it was therefore necessary to find so theiven by observations with an unassisted eye This necessity proation, published in 1800, in which, after drawing the distinction between absolute and intrinsic brightness, HERSCHEL gave an expression for the _space-penetrating power_ of a telescope The reasoning at the base of this conception was as follows
The ratio of the light entering the eye when directed toward a star, to the whole light given out by the star, would be as the area of the pupil of the eye to the area of the whole sphere having the star as a centre and our distance from the star as a radius If the eye is assisted by a telescope, the ratio is quite different In that case the ratio of the light which enters the eye to the whole light, would be as the area of the lass to the area of the whole sphere having the star as a centre and its distance as a radius Thus the light received by the _eye_ in the two cases would be as the area of the pupil is to the area of the object-glass For instance, if the pupil has a diameter of two-fifths of an inch, and the mirror a diaht would enter the eye when assisted by the telescope as when unarmed, since the _area_ of the pupil is one-hundredth the _area_ of the objective
If a particular star is just visible to the naked eye, it will be quite bright if vieith this special telescope, which makes it one hundred times more brilliant in appearance If we could move the star bodily away from us to a distance ten tihtness, as seen with the telescope, to what it was at first, as seen with the eye alone, _i e_, to bare visibility Moving the star to ten times its present distance would increase the surface of the sphere which it illuminates a hundred-fold We cannot htnesses, and thus (presuument was, then, as follows: Since with such a telescope one can see a star ten times as far off as is possible to the naked eye, this telescope has the power of penetrating into space ten times farther than the eye alone But this number ten, also, expresses the ratio of the diameter of the objective to that of the pupil of the eye, consequently the general law is that the _space-penetrating power_ of a telescope is found by dividing the diameter of the mirror in inches by two-fifths The diameter of the pupil of the eye (two-fifths of an inch) HERSCHEL determined by ood, however, provided no ht were lost by the repeated reflections and refractions in the telescope than in the eye That light must be so lost was evident, but no data existed for deter series of photo powers of the ht trans the corrections thus deduced experi power of his twenty-foot telescope, hich he es, was sixty-one times that of the unassisted eye, while the space-penetrating power of his great forty-foot telescope was one hundred and ninety-two times that of the eye In support of his important conclusions HERSCHEL had an almost unlimited amount of experimental data in the records of his observations, of which he made effective use
By far the most important of HERSCHEL'S work in the domain of pure physics was published in the saation of the space-penetrating powers of telescopes was undertaken for the sole purpose of aiding hi the dimensions of the stellar universe, and there was no temptation for him to pursue it beyond the lih the first hint leading to remarkable discoveries was a direct consequence of his astronomical work, the novelty and interest of the phenoation very far beyond the inated
Having tried lasses between the eye-piece of his telescope and the eye, in order to reduce the inordinate degree of heat and light transmitted by the instrument when directed towards the sun, he observed that certain coht to pass, but transmitted so much heat that they could not be used; while, on the other hand, different colasses would stop nearly all the heat, but allow an inconveniently great aht to pass At the saes of the sun were of different colors This suggested the question as to whether there was not a different heating power proper to each color of the spectrus of sensitive thermometers exposed in different portions of an intense solar spectru with the violet end, he ca before that of heat, which lay at the other extremity, that is, near the red By several experiments it appeared that the maximum of illumination, _i e_, the yellow, had little more than half the heat of the full red rays; and from other experiments he concluded that even the full red fell short of the maximum of heat, which, perhaps, lay even a little beyond the limits of the visible spectrum
”In this case,” he says, ”radiant heat will at least partly, if not chiefly, consist, if I ht; that is to say, of rays co from the sun, that have such a , as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the ible rays; as those which have greater or less momenta are likely to becoht”
In his second paper on this subject, published in the same year, HERSCHEL describes the experiiven above This paper contains a ree which admirably illustrates HERSCHEL'S philosophic ht, those rays which illuminate objects, and radiant heat, those which heat bodies, it ht be essentially different froest 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 oneIf this be a true account of the solar heat, for the support of which I appeal to my experiments, it remains only for us to adibility of those which are contained in the prisht, are adht and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat”
We no that the reasoning and conclusion here given are entirely correct, but they have for their basis only a philosophical conception, and not a series of experined especially to test their correctness Such an experimental test of this important question was the motive for a third and last paper in this department of physics
This paper was published in voluave the results of two hundred and nineteen quantitative experiments
Here we are at a loss to knohich to ad such accurate data with such inadequatesuch a nuested in the course of the investigation--or the intellectual power shown into a system such intricate and apparently self-contradictory phenomena It is true that this discussion led him to a different conclusion from that announced in the previous paper, and, consequently, to a false conclusion; but al lay in a principle which belongs to a later period of intellectual development than that of HERSCHEL'S own time
HERSCHEL made a careful deterht and of heat in the pris fact that not only where the light was at a maximum the heat was very inconsiderable, but that where there was a ht
”This consideration,” he writes, ”must alter the for thus at least partly decided, since it is ascertained that we have rays of heat which give no light, it can only beco raysobjects visible, superadded to their now already established power of heating bodies This being the case, it is evident that the _onus probandi_ ought to lie with those who are willing to establish such an hypothesis, for it does not appear that Nature is in the habit of using one and the same mechanism with any two of our senses Witness the vibration of air that makes sound, the effluvia that occasion smells, the particles that produce taste, the resistance or repulsive powers that affect the touch--all these are evidently suited to their respective organs of sense”
It is difficult to see how the fallacy of this argument could have been detected by any one not faical law that the nature of a sensation is in no wise deter it, but only by the character of the nerves acted upon; but, as already intis to a later epoch than the one we are considering HERSCHEL thus finally concluded that light and radiant heat were of essentially different natures, and upon this supposition he explained all of the phenomena which his numerous experiments had shown him So complete and satisfactory did this work appear to the scientific world, that for a long time the question was looked upon as closed, and not until thirty-five years later was there any dissent Then the Italian physicist, MELLONI, with instrumental means a thousand tier store of cognate phenoeneration which had elapsed, to serve as a guide, discovered the true law This, as we have seen, was at first adopted by HERSCHEL on philosophical grounds, and then rejected, since he did not at that time possess the key which alone could have enabled him to properly interpret his experiments
It is well to summarize the capital discoveries in this field made by HERSCHEL, more particularly because his claiely overlooked by historians of the developator, showed that radiant heat is refracted according to the laws governing the refraction of light by transparent media; that a portion of the radiation fro the sensation of vision, and that this portion is the less refrangible; that the different colors of the spectru powers, which are not proportional to their lureatly in their power of trans radiant heat, and that this power does not depend solely upon their color; and that the property of diffusing heat is possessed to a varying degree by different bodies, independently of their color
Nor should we neglect to emphasize, in this connection, the importance of his ht in the different portions of the solar spectrum It is the more necessary to state HERSCHEL'S clailected by those who should first have done him justice In his ”History of Physics,”
POGGENDORFF has no reference to HERSCHEL In the collected works of VERDET, long bibliographical notes are appended to each chapter, with the intention of exhibiting the progress and order of discovery But all of HERSCHEL'S work is overlooked, or indexed under the name of his son