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 investigations. 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 Royal in the Greenwich ‘ Seven-year Catalogue,’ are — 0''*035 in R.A., and +1",24 in N.P.D.
The parallax of Sirius from the observations of Henderson, corrected by Bessel, =0"-150. A recent investigation by Mr. C. Abbe f 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 lw,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, 1868.
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 frorii 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. t Monthly Notices of the Royal Astronomical Society, vol. xxviii. p. 2.
Since the diminished intensity of the marginal parts of the sun is probably due to the greater depth of solar atmosphere through which it had passed, it was conceivable that by its spectrum the light of these parts of the solar disk might exhibit some indications of the larger amount of absorbing medium which it had traversed.
Two different methods of observation were employed. The telescope, armed with one of the spectroscopes constructed for stellar observations, was directed to the sun. By the aid of the finder the telescope was moved so as to bring in succession upon the slit different parts of the solar disk. Though the detection of any difference depended upon the memory of the observer, still any considerable alteration would certainly have been noticed.
The other plan was to cause the sun’s light, after reflection from a plane mirror attached to a heliostat moved by clockwork, to fall upon an achromatic object-glass of 6 feet focal length, by which an image of the sun was formed upon the slit of the large spectroscope which I employed for the examination of the spectra of the chemical elements.
By neither of these methods have I been able to detect any difference in the spectra of different parts of the sun’s disk.
2. I have made numerous observations for the purpose of obtaining a view, if possible, of the red prominences which are seen during a solar eclipse. The invisibility of these objects at ordinary times is supposed to arise from the effect of the illumination of our atmosphere. If these bodies are gaseous, their spectra would consist of bright lines. With a powerful spectroscope the light from our atmosphere near the sun’s limb would be greatly reduced in intensity by the dispersion of the prisms, while the bright lines of the prominences, if such were present, would remain but little diminished in brilliancy. These observations have been made with different spectroscopes, and also with other contrivances arranged on the same principle, but hitherto without success.
[The observations made in India of the solar eclipse of August 18 have shown that the spectra of the prominences are discontinuous. Lieut. Herschel, R.E., who had charge of the instruments sent out by the Royal Society, determined approximately the position of three bright lines. One in the red about C, one apparently coincident with D, and one near F. On the day after the eclipse, M. Janssen, making use of a method similar to that described above, succeeded in observing the solar [protuberances. On October 20, 1868, Mr. Lockyer also succeeded in viewing the spectrum of a protuberance. Now that the positions of the lines are known, one of the “ other contrivances ” mentioned above will probably succeed. It consisted of screens formed of absorptive media, by which light of all refrangibilities other than those corresponding to the lines of the flames might be absorbed. By this method the flames could be examined and measured.—October 30,1868.]
8. The third object which I had proposed to myself was to seek to gain from an examination of the spectra of the umbrae and penumbrse of solar spots some information as to the nature of these remarkable phenomena.
I had already made some experiments in this direction when in August 1866 I received mdccclxviii. 4 a
