548 
ME. W. HUGGINS ON THE SPECTEA 
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 com- 
parison, 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 0T09 millionth of a millimetre. If the velocity of 
light be taken at 185,000 f miles per second, and the wave-length of F at 486-50 mil- 
lionths of a millimetre (Angstrom’s value is 486-52, Ditscheiner’s 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 earth’s 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 earth’s 
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 earth’s 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 earth’s motion will be greatest upon the light of a star situated in 
the plane of the ecliptic, and will decrease as the star’s latitude increases, until with 
* The value in wave-lengths of the divisions of the micrometer for different parts of the spectrum was deter- 
mined by the aid of the tables of the wave-lengths corresponding to every tenth line of Kjrchhoff’s map by 
Dr. Wolcott Gibbs (Silliman’s Journal, vol. xliii. January 1867). A paper on the same subject by the Astro- 
nomer Eoyal, presented to the Eoyal Society, is not yet in print. [The Astronomer Eoyal’s 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 Kirchhoff’s 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 Eoyal, of Ditscheiner’s 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 velo- 
city, when diminished in this ratio, agrees nearly with the result obtained by Foucault from direct experiment. 
