ASTRONOMY — HALE. 57 



important base of supplies like Los Angeles, will indicate the value 

 of this lease to the Solar Observatory. 



In Contributions from the Solar Observatory No. i , I have given a 

 detailed account of the systematic tests of the atmospheric conditions 

 at Mount Wilson made prior to January i, 1905. Since the tele- 

 scopic tests of this period were necessarily limited to solar observa- 

 tions with a refractor of $% inches aperture, they might perhaps be 

 considered inconclusive, especially as they related entirely to sharp- 

 ness of definition, and took no account of transparency of the atmos- 

 phere. At present, however, any doubts that might have existed 

 on this score have been removed, for extensive tests with powerful 

 instruments have fully confirmed, and even strengthened, the favor- 

 able opinion previously entertained. The large aperture and great 

 focal length of the Snow telescope have rendered the tests of the 

 solar definition very severe, and the number of days of fine seeing 

 recorded under such conditions is most gratifying. The transpar- 

 ency and uniformity of the sky by day have been fully proven by 

 Mr. Abbot's precise measures of the solar radiation with the pyrhe- 

 liometer and spectro-bolometer, and these conditions, with the long 

 stretches of unbroken clear weather, have left no doubt of the ad- 

 vantages to be gained from a continuation on Mount Wilson of the 

 important research instituted here by the Smithsonian Expedition. 

 Professor Barnard's remarkable photographs of the southern Milky 

 Way, including many star clouds and extended nebulae not previ- 

 ously recorded, and showing numerous stars to exist in regions here- 

 tofore supposed to be devoid of stars, testify to the transparency and 

 purity of the night sky, and his systematic tests of the seeing by 

 night have shown it to be very fine. These tests, made with the 

 5-inch guiding telescope of the Bruce telescope, have been amply 

 confirmed with the large aperture of the Snow telescope. In photo- 

 graphing stellar spectra with the high dispersion of a powerful 

 grating spectroscope, the observer has the star's image constantly 

 under observation for man}' hours at a time. In this work it has been 

 found that the average night-seeing is exceedingly good, while the low 

 wind- velocity, coupled with the transparency of the atmosphere, 

 afford additional advantages which should render Mount Wilson an 

 ideal site for the 5 -foot reflector. 



PLAN OF RESEARCH. 



Before proceeding to describe the work so far accomplished, it is 

 desirable to call attention to the plan of research prepared for the 

 observatory. This plan has been set forth in Contributions from 



