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 in a 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. Co well 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 reflecting 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 



