366 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1910. 



As to the laboratories proper at Pasadena, they also include all 

 that is necessary for physical and spectroscopic researches, which in 

 the minds of the founders should be the indispensable complement 

 of astronomical investigations. If in the laboratory, by known means, 

 the phenomena observed in the stars are reproduced, then, indeed, 

 the presumption is tenable that the phenomena take place in the stars 

 just as they do in our experiment. The proof is truly not rigorous, 

 and the conditions which prevail in the celestial objects are more 

 or less difficult to realize, and may at times result in unexpected con- 

 sequences. Nevertheless, it is the only method available in astro- 

 physics for attaining the truth, and we must leave to the future the 

 task of definite corroboration. We should add that this idea has 

 become very general, and that all astrophysical observatories have 

 more or less complete laboratories. 



Since its construction the establishment at Mount Wilson has been 

 open to astronomers of all nations who have wished to work or study 

 there. And so we had the pleasure of meeting our colleague, M. H. 

 Chretien, of the Observatory of Nice, who had been studying there 

 some eight months. So, also, W. H. Julius was enabled to study in 

 application to the sun his well-known theory of anomalous disper- 

 sion set forth in this review in 1903. With this theory he explains 

 a great many phenomena, principally those of the chromosphere and 

 prominences by reason of the curvilinear paths of the light rays in 

 the solar atmosphere. Kapteyn, also from Holland, has come to 

 Mount Wilson for the purpose of continuing his researches upon the 

 absorption of light in celestial space. The differences which he has 

 noted between the visual and the photographic magnitudes of the 

 stars, which become larger the greater the distances of the stars, may 

 be easily explained by the absorption of the light by some cosmic 

 medium. 



We have considered the general organization of this observatory; 

 let us now discuss the instruments. The first one put into service 

 was the Snow telescope (pi. 5), which does not differ materially 

 as a whole from that used at Meudon for the study of the solar at- 

 mosphere. It consists of a two-mirror coelostat which sends the 

 solar beam horizontally to a great spectrograph and a spectrohelio- 

 graph. The latter, which is of moderate dispersion, serves to take 

 plates of the sun in monochromatic light several times a day. With 

 it was explored the mean layer of chromospheric hydrogen, in which 

 are seen at times, besides the more important filaments of the upper 

 stratum, isolated later at Meudon, the peculiar more or less vortical 

 movements that bring to mind the classic experiment of the magnetic 

 spectrum. 



The instrument next in order to the " Snow " is the tower tele- 

 scope, which is designed on a totally different plan. In order to 



