108 THE ROYAL SOCIETY OF CANADA 
All the measures above referred to were obtained visually and, 
although Jewell made some photographic observations at Baltimore, 
no results were published. The practical inception of the photographic 
method with its many advantages is due to Adams at Mt. Wilson who, 
using the Snow and tower telescopes in 1906-7 and in 1908, with sta- 
tionary spectrographs of high dispersion, obtained results of remark- 
able accuracy with probable errors several times lower than those of the 
earlier observations. He used the region in the violet between À 4200 
and 4 4300 with a linear dispersion 0-7 and 0-6. Angstrom to the milli- 
metre. His results indicate that the rotation rate is practically con- 
stant, thus differing from Halm. The value obtained from different 
elements varies slightly seeming to depend upon their height in the 
reversing layer, and this is particularly shown for the H, line and for 
Ca 4227, whose rotational velocities are about 3 % greater than that 
of the general reversing layer. 
At the meeting of the International Union for Co-operation in 
Solar Research, held at Mt. Wilson last August, which one of us had 
the privilege of attending, a committee consisting of Messrs. Adams, 
Belopolsky, Dyson, Newall, Plaskett and Schlesinger was appointed, 
the question was carefully discussed and arrangements made for a 
combined attack on the various phases of the problem. To this end 
selected regions of the spectrum, between À 3600 and À 6400, were 
allotted to each member for observation, and in addition a general 
region, that already used by Adams, between À 4220 and À 4280 was 
chosen for universal observation to serve as a check upon instrumental 
or personal errors. The region allotted to the Dominion Observatory 
is from À 5500- À 5700, the latitudes to be observed 0°, 15°, 30°, 45°, 
60° and 75°, to which are to be added, if possible, the higher latitudes 
80° and 85°. In this region, lines are to be selected from as many 
elements as possible, especially those of high or low atomic weight, in 
order to throw light upon the question of differences in rotational value 
for different elements. 
A description of the coelostat telescope employed to produce the 
sun’s images examined in the present work, is given in the Report of 
the Chief Astronomer.! 
This instrument is mounted in a louvred house (part of which 
may be rolled back to admit the sunlight) which is connected by means 
of a louvred passage to a tunnel which enters the Solar Research 
Laboratory on the north side of the Observatory. It consists essen- 
tially of a plane silvered mirror twenty inches in diameter mounted 
! Plaskett, Report of the Chief Astronomer for the year ending March 31, 1909, 
pp. 207-209. 
