I made some attempts to compare the strong line at C with the corresponding line of hydrogen; but when the large spectroscope was employed, though the lines could be seen with tolerable distinctness, they were not bright enough to admit of a trustworthy determination of their relative position. When one of the compound prisms was removed, the lines were much more easily seen, but under these circumstances the amount of dispersion was insufficient for my present purpose.
The lines of Sirius which, in conjunction with Dr. Miller, I had compared with those of iron, magnesium, and sodium are not sufficiently well seen in our latitude for comparison, when a powerful train of prisms is employed, such as is necessary for this special inquiry.
From these observations it may, I think, be concluded that the substance in Sirius which produces the strong lines is really hydrogen, as was stated by Dr. Miller and myself in our former paper. Further, that the aggregate result of the motions of the star and the earth in space, at the time when the observations were made, was to degrade the refrangibility of the line in Sirius by an amount corresponding to 0-040 of the micro-meter-screw. Now the value of the wave-lengths of 0-01 division of the micrometer at the position of F is 0,02725 millionth of a millimetre*. The total degradation of refrangibility observed amounts to 0409 millionth of a millimetre. If the velocity of light be taken at 185,000f miles per second, and the wave-length of F at 486-50 millionths of a millimetre (Angstroms value is 48652, Ditscheiners 486-49), the observed alteration in period of the line in Sirius will indicate a motion of recession existing between the earth and the star of 41-4 miles per second.
Of this motion a part is due to the earths motion in space. As the earth moves round the sun in the plane of the ecliptic, it is changing the direction of its motion at every instant. There are two positions, separated by 180, where the effect of the earths motion is a maximum, namely, when it is moving in the direction of the visual ray, either towards or from the star. At two other positions in its orbit, at 90 from the former positions, the earths motion is at right angles to the direction of the light from the star, and therefore has no influence on its refrangibility.
The effect of the earths motion will be greatest upon the light of a star situated in the plane of the ecliptic, and will decrease as the stars latitude increases, until with
* The value in wave-lengths of the divisions of the micrometer for different parts of the spectrum was determined by the aid of the tables of the wave-lengths corresponding to every tenth line of Kikchhoffs map by Dr. Wolcott Gibbs (Sillimaits Journal, vol. xliii. January 1867). A paper on the same subject by the Astronomer Royal, presented to the Royal Society, is not yet in print. [The Astronomer Eoyals paper is contained in the Philosophical Transactions for 1868, Part I. p. 29. The wave-lengths computed by him differ slightly from those assigned to Kirchhobts numbers by Dr. Gibbs at the part of the spectrum under consideration in the text. The difference is due in part to the employment, by the Astronomer Royal, of Ditscheinebs later measures. These give for F the higher value of 486'87.October 1868.]
t The new determination of the value of the solar parallax by observations of Mars requires that the usually received velocity of light, 192,000 per second, should be reduced by about the one-twenty-seventh part. The velocity, when diminished in this ratio, agrees nearly with the result obtained by I'oucAr lt from direct experiment.
respect to a star situated at the pole of the ecliptic, the earths motion during the whole of its annual course will be perpendicular to the direction of the light coming to us from it, and will be therefore without influence on its period.
That part of the earths resolved motion which is in the direction of the visual ray, and which has alone to be considered in this investigation, may be obtained from the following formula:
Earths motion towards star=v.cos X.sin (II'),
where v is earths velocity, I the earths longitude, I' the stars longitude, and X the stars latitude.
At the time when the estimate of the amount of alteration of period of the line in Sirius was made, the earth was moving from the star with a velocity of about 12 miles per second.
There remains unaccounted for a motion of recession from the earth amownting to 29*4 miles per second, which we appear to be entitled to attribute to Sirius.
It may be not unnecessary to state that the solar motion in space, if accepted as a fact, will not materially affect this result, since, according to M. Otto Struves calculations, the advance of the sun in space takes place with a velocity but little greater than one-fourth of the earths motion in its orbit. If the apex of the solar motion be situated in Hercules, nearly the whole of it will be from Sirius, and will therefore diminish the velocity to be ascribed to that star.
It is interesting, in connexion with the motion of Sirius deduced from these prismatic observations, to refer to the remarkable inequalities which occur in the rather large proper motion of that star. In 1851 M. Peters* showed that the variable part of the proper motion of Sirius in right ascension might be represented by supposing that Sirius revolves in an elliptic orbit, round some centre of gravity without itself, in a period of 50-093 years. This hypothesis has acquired new interest, and seems indeed to have received confirmation from direct observation by Alvax Clarks discovery of a small companion to Sirius.
Professor Safford 'f and Dr. AuwersJ have investigated the periodical variations of the proper motion of Sirius in declination, and they have found that these variations, equally with those in right ascension, would be reconcileable with an elliptic orbital motion round a centre not in Sirius. The close coincidence of the observed positions of the new satellite with those required by theory, seem to show that it may be the hypothetical body suggested by Peters, though we must then suppose it to have a much greater mass relatively to Sirius, than that which its light would indicate.
At the present time the proper motion of Sirius in declination is less than its average
* Astron. Nachrichten, No. 748.
t Proceedings of the American Academy, rvol. vi.; also Astron. Notices, Ann. Arbor, No. 28; Monthly Notices, vol. xxii. p. 145.
t Astron. Nachrichten, No. 1506; Monthly Notices, vol. xxii. p. 148, and vol. xxv. p. 39.
