SECTION III., 1911. [107] Trans. R.S. C. 
Methods and Preliminary Results in the Spectroscopic Determination 
of the Solar Rotation. 
By J. 8. PLASKETT, B.A. F.R.S.C. anp R. E. DeLury, Px.D. 
(Read May 17, 1911.) 
The determination of the rotation of the sun by the Doppler dis- 
placement of the spectral lines at opposite limbs was early placed on 
the programme of work of the Dominion Observatory; was indeed 
one of the main investigations planned for the equipment for solar 
research whose nucleus was the 20-inch coelostat used in the Canadian 
Eclipse Expedition to Labrador in 1905. But various causes have 
delayed the actual inception of effective observation, the principal 
ones being the delay in the construction of the shelter for the horizontal 
coelostat telescope and the difficulty of securing a suitable plane grating 
for the spectrograph. No less than four different gratings have been 
tested and even the one finally in use it is hoped eventually to replace 
by a better one. 
Before describing the apparatus, it will be of value to give a brief 
historical sketch of the work already done on this problem. The 
early work of Vogel in 1871, was of use chiefly in showing the correct- 
ness of the Doppler principle and in indicating the possibility of obtain- 
ing by more powerful apparatus an accurate value of the solar rotation. 
Young, in 1876, with a diffraction grating spectroscope measured the 
displacement of the lines at the sun’s equator, showing that the velocity 
of rotation thereby obtained agreed with that determined from the 
observation of sunspots. Duner at Upsala, in 1888, began his classical 
researches upon the solar rotation continued through 1889 and 1890, 
and repeated in the years 1901 to 1903. His observations made with 
a grating spectrograph attached to a refractor consisted of visual 
measures of the displacements of an iron line 4 6301-718 at the sun’s 
limb with respect to a neighboring atmospheric line À 6302-209. . The 
former is displaced by the solar rotation while the latter, due to absorp- 
tion in the earth’s atmosphere, is stationary. The measures are, there- 
fore, wholly differential, the values obtained were of high accuracy 
and Duner’s methods have formed the basis of more recent investi- 
gations. Halm at Edinburgh in 1903 to 1906, using the same lines as 
Duner, measured visually the solar rotation and obtained results of 
great accuracy, which seemed to indicate a variability in the rotation 
rate, one of the important problems to be settled by future investiga- 
tion. 
Sec. III., 1911. 8. 
