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And again-" Now see B (95)
find it no longer white, but a strong blue!" And again
Herculis; according to Smyth one is greenish and the other red; but we think them both a golden yellow. A Ophiuchi, by Smyth, one ruddy and the other pale yellow; but we take them to be both orange. The contrary occurs in Bootis, the components of which by Smyth are both pale yellow; but we deem one to be orange and the other azure."
Under the circumstances to which I have already alluded, I am not at this stage disposed to theorize on the objects thus brought into juxta-position: and the colours of double-stars must be much more accurately assigned, and more ably experimented upon, before we can admit that the nature and character of those suns can possibly change in short periods. change in short periods. Sir David Brewster observes, that there can be no doubt that in the spectrum of every coloured star certain rays are wanting which exist in the solar spectrum; but we have no reason to believe that these defective rays are absorbed by any atmosphere through which they pass. And in recording the only observation perhaps yet made to analyse the light of the coloured stars, he says "In the orangecoloured star of the double-star 3 Herculis, I have observed that there are several defective bands. By applying a fine rock-salt prism, with the largest possible refracting angle, to this orange star, as seen in Sir James South's great achromatic refractor, its spectrum had the annexed appearance (in the Campden Hill Journal), clearly shewing that there was one defective band in the red space, and two or more in the blue space. Hence the colour of the
star was orange, because there was a greater defect of blue than of red rays.
In the present incertitude, it is suggested that variations in colour may be owing to variations in stellar velocity; but in this case would there not also be as palpable a variation in brightness? If it shall be found that the tints actually vary, the comparative magnitude should also be carefully noted, to establish whether a variability in brightness accompanies the changes of colour. Sestini, however, does not view the matter in this light: he holds that the undulations of each colour arrive in succession to our eyes, and that therefore at last, when they have all reached us, they will appear white. In
arguing the circumstances necessary for the case-as the strength of vibrations, with their number and velocity in a given time-he cites Huyghens, Euler, Young, Fresnel, and Arago. Quoting Herschel's data he observes, that five hundred and thirty-six billions of vibrations cause us to see yellow, whilst six hundred and twenty-five billions exceed the number that shews blue: that is, when the tangential celerity of the moving star in relation to its companion comes at its maximum to equal one-thirteenth of that of light. Its green colour will change insensibly into yellow on increasing its distance, and then, receding through the same steps, it will again become green; beyond which, as it approaches the eye, it will become a full blue; finally, in the inverse order, it will return to green, and so on. But if we accelerate the velocity of the star to one-fifth of that of light, we shall have the number of vibrations corresponding to red = four hundred and eighty-one billions, and seven hundred and twenty-one billions, which exceeds that of violet. In this supposition, the green star when furthest from its companion will become red, and when approaching it must be of an intensely strong violet tinge; after which, owing to its circular orbit, it will in receding again become green, thus passing through all the colours of the spectrum. These are the ratios
Admitting these and the like grounds, as the laws of new stars and binary systems may be somewhat elucidated thereby, I strongly recommend repeated examinations of the brightness and colours of stars to the well-equipped amateur, who is also happily possessed of a good eye, perseverance, and accurate notation. But even thus prepared, I would advise him, before entering upon the undertaking, to study well the third chapter of the great work of my highly-esteemed friend Sir John Herschel, on the Uranography of the Southern Hemisphere: it treats of Astrometry, or the numerical expression of the apparent magnitudes of the stars. In a more advanced state of this question, the measurement of brightness should always accompany that of colours, since
a change in the one might possibly produce variation in the other: and who can say that numerical measures may not yet be made with such precision that the distance of stars thereby may be given? The observer must not however be unnerved by the difficulties, some of them apparently insuperable, which beset the inquiry: nor by the philosopher's assertion that "nothing short of a separate and independent estimation of the total amount of the red, the yellow, and the blue rays in the spectrum of each star would suffice for the resolution of the problem of astrometry in the strictness of its numerical acceptation; and this the actual state of optical science leaves us destitute of the means even of attempting with the slightest prospect of success." This is indeed a damper to our argument, so far at least as stars differing in colour are concerned; but perseverance in a good cause has often been rewarded with marvellous accomplishments,-and it is well to remember that
These remarks will
objection to the facts
By many blows that work is done,
hardly be impinged upon in practice by taking one upon which Sestini's theory is founded, namely, the velocity of the stars; since, in the present day-even admitting proper motions and translations in space to their fullest extent-it is not necessary to consider the possible rate of sidereal movements as capable of bearing any sensible ratio to the speed of light. In citing the case of the orbital velocity of the companion of a double star, he should have applied it to a Centauri, an object of which we know all the elements, its distance from us and from each other in miles, the mass of the components as compared to our Sun, their quantity of light as compared to the same, and the periodic time;-all these we know to a greater degree of confidence than in any other similar body. Now the theory fails upon this test; for the mean orbital velocity of the companion may be assumed as 2.5 miles per second, while Sestini's limits of and of 13 the velocity of light would make it fifteen thousand and thirty-eight thousand miles in the same time. The velocity of light assumed here is, however, it
must always be remembered, that of the Sun; - that determined by direct observations of the solar orb itself, or by the eclipses of Jupiter's satellites, whose reflections still give us solar light, and traversing the same medium, whatever it be, filling the planetary spaces. But we may reasonably expect, and, indeed, the experiments detailed above, on the spectra of different stars, appear to indicate, if not actually to prove, that the light of some of the stars is absolutely of a distinct nature, and radically of a different composition, to that of the Sun; while the media which the rays have to pass through, may be of a kind unknown in any part of the whole extent of our planetary circles, and of a nature of which we are at present profoundly ignorant.
Evidently, therefore, when the speed of transmission of the stellar rays comes into play, we may have to deal with velocities very different to that on which our correction for aberration is founded. Granting that however, and to the widest extent, extending even the somewhat doubtful experiments which have been made on the velocity of electric light as compared with the solar, and on the transmission of ordinary light through air and through the denser medium of water, still there is nothing as yet to shew, that we are likely to meet with any kind of light moving at so slow a rate, as to bear the proportion which Sestini's theory requires to the actual speed at which any star has been found to move.
There is, however, another way in which the peculiar habitudes of rays of light may produce a difference of colour in a star, and make it even run through the whole of the colours of the spectrum from one end to the other and back again, in a greater or less space of time according to the particular circumstances of the case. This will occur if the different coloured rays of which the white beam is composed, have intrinsically in themselves, or by reason of the nature of the medium which they traverse, any difference in the velocity of their transmission.
According to the Newtonian doctrine of "emission," the separate colours are actually produced by different degrees of velocity. But, according to the "undulating" theory, which has since been shown by Young and
Fresnel to be far more probable than the other, if not really to be the true theory, the various tints are produced by means of undulations of different lengths, and they have even been able to measure the lengths of these minute waves, and have assigned them as follows:
Now, though this by itself may say nothing with respect to the rapidity with which each undulation may be transmitted, it renders the probability of such a difference extremely great; and, though that difference be so very small that there is no hope of ever being able to make it manifest in any scientific apparatus of even the most delicate description, yet, on account of the great distance of the stars, the effect may become at length very sensible. For although the difference of the rate of propagation of the waves of each ray may be the smallest conceivable quantity, yet, if that different rate be kept up during the whole one thousand years that we suspect must be occupied by the light of some of the stars in reaching us, notwithstanding that it may travel on the average one hundred and ninety-two thousand miles in a second, it is manifest that, after continuing to grow during so great a length of time, a very decided effect may at last be produced.
If a new star suddenly appears in some part of the sky, the rays of light immediately travel off to announce the fact everywhere, and to us amongst the number of other orbs; and it matters not whether the light consist in the emission of particles, or the propagation of waves of different orders, as many of Arago's "couriers as there are different colours in the spectrum are sent off with the intelligence; and, if one is able to accomplish the great intervening distance between the star and us in a less space of time than the others, and so arrive before them, we shall see the star of that colour first, say blue. In that case the next to arrive would be the yellow, and then arriving and