262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911, 
method and is probably considerably less in quantity than that 
present in the earth’s atmosphere above the summit of Mount 
Whitney. ; 
The question of the suitability of Venus for organic life seems to 
depend upon the determination of its rotation period. If, as is now 
mostly believed, it always turns the same face to the sun then the 
one side will be baked and the other frozen. If, on the other hand, 
it turns on its axis in about 24 hours, then it is practically certain 
to be ina condition to support life. The only possible test between the 
two theories is the spectroscopic one, as in the solar rotation, by 
observing the line shift at opposite limbs. In this case, however, 
we have difficulties owing to the bad seeing at the comparatively low 
altitude of Venus and the disturbance of the image, so that it is diffi- 
cult to determine in what region of the planet the spectra were made. 
The advent of Halley’s Comet proved possibly as disappointing to 
astronomers as to the general public, for it did not show many 
unusual features, and not much additional knowledge concerning the 
nature of comets was obtained. The motion of a detached part of the 
tail, as determined from three photographs at Williams Bay, Hono- 
lulu, and Beirut, showed the presence of an accelerating force, as its 
velocity relative to the head increased from 23 miles to 37 miles a 
second in seven or eight hours. To my mind the most remarkable 
feature of its return was the accuracy of the computation so success- 
fully carried through by Messrs. Cowell and Crommelin, in which they 
predicted its perihelion passage within less than three days. When 
considered in connection with the large number of disturbing ele- 
ments to be taken into account and the exceedingly complex and 
cumbersome calculations required, their ephemeris was a marvelous 
piece of work, and they well deserved the recognition it received. 
Before discussing some of the advances in our knowledge of the 
sidereal universe it has seemed desirable to refer to the improvements 
effected in apparatus for observation. At the head comes naturally 
the large refiecting telescope with a mirror of 60 inches diameter, 
recently installed on the summit of Mount Wilson, California, at an 
elevation of 5,886 feet. This telescope was designed and the mirror 
was figured by Prof. G. W. Ritchey, superintendent of instrument 
construction, of the Solar Observatory, who also is doing much of 
the photographic work with the telescope. I had the privilege of 
carefully examining the mechanism and of observing with the tele- 
scope, and it is certainly a superb instrument. The optical proper- - 
ties are practically perfect; and the difficulty of temperature changes, 
the most troublesome met with in reflectors, has been successfully 
overcome. The mechanical construction is also unexcelled, and the 
instrument, although its moving parts weigh 23 tons, drives with the 
greatest smoothness and ease. The most magnificent photographs 
