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It appeared premature at the time to refer to these negative results, as it did not seem to be probable that the stars were moving with velocities sufficiently great to cause a change of refrangibility which could be detected with our instrument. The insufficiency of our apparatus for this very delicate investigation does not, however, diminish the trustworthiness of the results we obtained respecting the chemical constitution of the stars, as the evidence for the existence or otherwise of a terrestrial substance was made to rest upon the coincidence, or want of coincidence in general character as well as position of several lines, and not upon that of a single line.
According to the undulatory theory, light is propagated with equal velocity in all directions, whether the luminous body be at rest or in motion. The change of refrangibility is therefore to be looked for from the diminished or increased distance the light would have to traverse if the luminous object and the observer had a rapid motion towards or from each other. The great relative velocity of light to the known planetary velocities, and to the probable motions of the few stars of which the parallax is known, showed that any alterations of position which might be expected from this cause in the lines of the stellar spectra would not exceed a fraction of the interval between the double line D, for that part of the spectrum.
I have devoted much time to the construction and trial of various forms of apparatus with which I hoped to accomplish the detection of so small an amount of change of refrangibility. The difficulties of this investigation I have found to be very great, and it is only after some years that I have succeeded in obtaining a few results which I hope will be acceptable to the Koyal Society.
The subject of the influence of the motions of the heavenly bodies on the index of refraction of light had already, at the time of the publication of our paper in 1864, occupied the attention of Mr. J. C. Maxwell, F.RS., who had made some experiments in an analogous direction. In the spring of last year, at my request, Mr. Maxwell sent to me a statement of his views and of the experiments which he had made. I have his permission to enrich this communication with the clear statement of the subject which is contained in his letter, dated June 10, 1867.
In 1841, Doppler showed that since the impression which is received by the eye or the ear does not depend upon the intrinsic strength a,nd period of the waves of light and of sound, but is determined by the interval of time in which they fall upon the organ of the observer, it follows that the colour and intensity of an impression of light, and the pitch and strength of a sound will be altered by a motion of the source of the light or of the sound, or by a motion of the observer, towards or from each other*.
Doppler endeavoured by this consideration to account for the remarkable differences of colour which some of the binary stars present, and for some other phenomena of the heavenly bodies. That Doppler was not correct in making this application of his theory is obvious from the consideration that even if a star could be conceived to be moving with
* “ Ueber das farbige Licht der Doppelsteme uud einiger anderer Gestime des Himmels,” Bohm. Gesell. Abh. ii. 1841-42, s. 465.
a velocity sufficient to alter its colour sensibly to the eye, still no change of colour would be perceived, for the reason that beyond the visible spectrum, at both extremities, there exists a store of invisible waves which would be at the same time exalted or degraded into visibility, to take the place of the waves which had been raised or lowered in refrangibility by the star’s motion. No change of colour, therefore, could take place until the whole of those invisible waves of force had been expended, which would only be the case when the relative motion of the source of light and the observer was several times greater than that of light.
In 1845, Ballot published a series of acoustic experiments which support Doppler’s theory in the case of sound. In the same paper Ballot advances several objections to Doppler’s application of his theory to the colours of the stars*.
This paper was followed by several papers by Doppler in reply to the objections which were brought against his conclusions f.
In 1847 two memoirs were published by Sestinj on the colours of the stars in connexion with Doppler’s theory J.
More recently, in 1866, Klinkerfues § published a memoir on the influence of the motion of a source of light upon the refrangibility of its rays, and described therein a series of observations from which he deduces certain amounts of motion, in the case of some of the objects observed by him.
The method employed by Klinkerfues has been critically discussed by Dr. Sohncke ||.
It may be sufficient to state that as Klinkerfues employs an achromatic prism, it does not seem possible, by his method of observing, to obtain any information of the motion of the stars; for in such a prism the difference of period of the luminous waves would be as far as possible, annulled. It is, however, conceivable that his observations of the light when travelling from E. to W., and from W. to E., might show a difference in the two cases, arising from the earth’s motion through the ether.
Father Secchi has quite recently called attention to this subject^]". In his paper he states that he has not been able to detect any change of refrangibility in the case of certain stars, of an amount equal to the difference between the components of the double line D. These results are in accordance with those obtained by myself and Dr. Miller in 1863, so far as they refer to the stars which had been examined by us.
Father Secchi’s method of using an unrefracted image as a fiducial mark with diverging rays passing through the prisms might, it is conceivable, be open to objection.
* “ Akustische Yersuche auf der Niederlandischen Eisenbahn nebst gelegentlichen Bemerkungen zur Theorie des Hrn. Prof. Doppler,” Pogg. Ann. B. lxvi, s. 321.
f See Pogg. Ann. B. lxxxi. s. 270, and Pogg. Ann. B. lxxxvi. s. 371.
X Memoria sopra i colori delle stelle del catalogo de Baily osservati dal P. Band Sestini. Boma, 1847.
§ “ Fernere Mittheilungen liber den Einfluss der Bewegung der Lichtquelle auf die Brechbarkeit eines Strahls, von W. KLnrcERFTTES,” Nachr. K. G. der W. zu Gottingen, No. 4, s. 33.
|| “ Ueber den Einfluss der Bewegung der Lichtquelle auf die Brechung, kritische Bemerkungen zu der Ent-deckung des Hrn. Prof. Klustzerfues. Yon Hrn. Dr. Sohjstcke, Astron.” Nadir. No. 1646.
IT Comptes Bendus, 2 Mars 1868, p. 398.