550 
MR. W. HUGGINS ON THE SPECTRA 
amount by nearly the whole of that part of it which is variable. May not this smaller 
apparent motion be interpreted as showing that a part of the motion of the star is now 
in the direction of the visual ray 1 This circumstance is of much interest in connexion 
with the result arrived at in this paper. 
Independently of the considerations connected with the variable part of the star’s 
proper motion, it must not be forgotten that the whole of the motion which can be 
directly observed by us is only that portion of its real motion which is at right angles 
to the visual ray. Now it is precisely the other portion of it, which we could scarcely 
hope to learn from ordinary observations, which is revealed to us by prismatic investi- 
gations. By combining the results of both methods of research we may perhaps expect 
to obtain some knowledge of the real motions of the brighter stars and nebulae. 
It seems therefore desirable to compare with the result obtained by the prism, the 
motion of Sirius which corresponds to its assumed constant proper motion. The values 
adopted by Mr. Main*, and inserted by the Astronomer Boyal in the Greenwich ‘ Seven- 
year Catalogue,’ are — 0' ,- 035 in R.A., and +l"-24 in N.P.D. 
The parallax of Sirius from the observations of Henderson, corrected by Bessel, 
= 0"T50. A recent investigation by Mr. C. ABBEf gives for the parallax the larger 
value of 0"-27. If the radius of the earth’s orbit be taken at its new value of 91,600,000 
miles, the assumed annual constant proper motion in N.P.D. of 1 ,,_ 24 would indicate, 
with the parallax of Henderson, a velocity of Sirius of 24 miles nearly per second, with 
the larger parallax of Mr. Abbe, a velocity of 43‘2 miles per second. It may be that in 
the case of Sirius we have two distinct motions, one peculiar to the star, and a second 
motion which it may share in common with a system of which it may form a part. 
Observations and comparisons, similar to those on Sirius, have been made on a Canis 
Minoris, Castor, Betelgeux, Aldebaran, and some other stars. I reserve for the present 
the results which I have obtained, as I desire to submit these objects to a reexamination. 
It is seldom that the air is sufficiently favourable for the successful prosecution of this 
very delicate research. 
§ V. Observations of the Sun. 
Received April 30, 1 868. 
I have recently applied the large spectroscope described in this paper with some 
success to the examination of the spectrum of the umbra of a large sun-spot. 
Before I describe the results of this examination, I will refer briefly to observations of 
the sun which I have made on many occasions, since 1864, with three distinct objects 
in view. 
1. I have sought to discover if the spectrum of the light from near the sun’s limb 
differs in any sensible degree from that of the light from the central parts of the disk. 
* Memoirs of the Royal Astronomical Society, vol. xix. 
f Monthly Notices of the Royal Astronomical Society, vol. xxviii. p. 2. 
