156 REPORT—1868. 
hydrogen, had been satisfactorily determined by myself and Dr. Miller in 
1863. We found also in the star lines coincident with some of those of iron, 
magnesium, and sodium. Of all these lines, the one which corresponds to F 
of the solar spectrum was alone seen with the steady distinctness necessary 
for comparison, when the powerful train of prisms was used. 
We have sufficient proof from our observations referred to, that this line 
is produced by hydrogen; at the time, therefore, of its formation at the 
star, its period would be identical with that of the corresponding line of 
hydrogen. 
This line in the star appears rather broader than in the solar spectrum ; 
it includes, perhaps, a range of wave-length about equal to that of the sepa- 
ration of the double line D; it is in a small degree nebulous at both edges. 
As the corresponding bright line of the spectrum of hydrogen, when the 
spark is taken in hydrogen at the pressure of the atmosphere, is too expanded 
for accurate comparison, the hydrogen was employed as it exists in the so- 
called vacuum-tubes. In this state the hydrogen gives a narrow and well- 
defined line. 
The result of a great many comparisons on many nights is to show that 
the bright line of hydrogen is, in a small degree, more refrangible than the 
dark line in the star (see fig. 4, Pl. V.). The amount of degradation in 
wave-length which the line had suffered, was found to be equal to 0-109 mil- 
lionth of a millimetre. 
If the velocity of light be taken at 185,000 miles per second, and the 
wave-length of F at 486:50 millionths of a metre, the alteration in period 
observed in the line of Sirius will indicate a motion of recession 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 the refrangibility of its light. 
The position of the star is also of importance ; for 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 respect 
to a star at the pole of the ecliptic, the earth’s 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 the period of the light. 
Now at the time the observations on Sirius were made, the earth was 
moving from the star with a velocity of twelve miles per second. There 
remains, therefore, a motion from the earth of 29-4 miles per second, which 
appears to belong to the star itself. 
The solar motion in space will not materially affect this result; for, ac- 
cording to Struve, the sun advances in space with a velocity but little 
greater than one-fourth of the earth’s orbital motion. If the apex of the 
solar motion be situated im 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 to remark that at the present time the proper motion of 
Sirius in declination is less than its average amount by nearly the whole of 
that part of it which is subject to variation. It may be that a greater part 
of the star’s motion is now in the direction of the visual ray. 
