38 MOUNTAIN OBSERVATORIES IN AMERICA AND EUROPE. 



kept in full activity since that time. Every class of observing Las re- 

 ceived attention — double stars, observations of the moon, planet.s, and 

 satellites, stellar spectra, pliotograpliic star-charts, zodiacal light obser- 

 vations, etc., and thus the materials for a judgment on the advantaavs 

 of the station are at hand. Professor W. II. Pickering's conclusions 

 are, in brief : 



The transparency of the sky was such that it was a common occurrence to see 

 third-magnitude stars set below the horizon where it was on the level with the 

 eye. 



With the 13-inch telescope ten and twelve diffraction-rings have been counted 

 under favorable circumstances around the brighter stars, each ring being nearly 

 if not absolutely motionless. 



Powers of 1140 diameters have been used to advantage on Venus in 

 the daytime, and the phases of Jupitei-'s satellites are seen as they enter 

 the planet's shadow at eclipse. This phenomenon has probably never 

 before been seen with a 13-inch telescope.* 



The sky is always clear in the dry season, and during most of the 

 mornings of the rainy season from November to April or May. 



There can be no doubt that the astronomical observatory at Arequi])a 

 is far more favorably situated as to observing conditions than most 

 permanent observatories. There is no (piestion that its skies are more 

 transparent, and the stars more stead)' than at fixed observatories in 

 the eastern parts of the United States, for example, or than most ol.)- 

 servatories in England and on the continent of Europe. It is more 

 difficult to obtain an accurate comparison between the steadiness of 

 the atmosphere at Arequipa and at California observatories as ^[t. 

 Hamilton, Mt. Wilson, and Echo Mountain. The Harvard College 

 Observatory maintained an observing station for some years on Mt. 

 Wilson, and it is the verdict of the observers, I believe, that Arequipa 

 is superior both in respect of transparent air and of steady definition. 

 A few comparisons have been made by astronomers of the Lifl< 

 Observatory between the conditions at Mt. Wilson and at Mt. Hamil- 

 ton, witli the general result that the best seeing at the two places is 

 practically of the same excellence. f This would make the best con- 

 ditions at Arequipa superior to those at Mt. Hamilton. A direct com- 

 parison between the two places was made by Professor Schaeberle 

 of the Lick Observatory in 1893. 



Professor Schaeberle stayed at Arequipa about two weeks (in tlie 

 latter part of May), and with regard to the transparency of the sky ob- 



* It is always seen at Mount Hamilton with the 36-inch equatorial.— E. S. H. 

 f Professor Barnard says of Mt. Wilson (July 18, 1892) that he has "nowhere 

 seen a finer, clearer sky." 



