REPORT OF COMMITTEE ON OBSERVATORIES 1 59 



mittee, I think that this covers all fields of astrophysics in an ex- 

 haustive way, and that hardly anything of importance can be added 

 to it. I beg briefly to call attention to a few special investigations 

 which, in my opinion, deser\^e special consideration : 



( 1 ) Investigation of the atmospheric lines of the solar spectrum 

 by simultaneous observations at both stations under the most diverse 

 atmospheric conditions and at all times of the j^ear. 



(2) Determination of the extinction of starlight by simultaneous 

 photometric observ^ations at both stations. 



(3) Thorough investigation of the photographic extinction by 

 simultaneous photographs of given groups of stars. 



(4) Determination of the absorption for various regions of the 

 solar spectrum by spectro-photometric measures. 



(5) Comparison of the light of the sun with that of the moon, the 

 planets, and the fixed stars, to determine by continued obseni'ations 

 whether any variation in the intensity of sunlight can be detected. 



In conclusion, I heartily wish you success in the great undertak- 

 ing, and trust that your plans will soon develop in the interests of 

 our science. 



[^From Professor J. Harhnann, Royal Astrophysical Observatory , Pots- 

 dam, Germa?iy.'\ 



[Translation.] 



April 12, 1903. 

 Let me extend to you my heartiest congratulations on the aston- 

 ishing advances which astrophj'sics has already made in America, 

 It must be acknowledged without en\'y that the new continent has 

 wholl}^ outstripped the old one, and I would regard the establish- 

 ment of a high altitude obser\'atory as a glory to American science. 

 When, as here in Potsdam, one is forced to observe with a great re- 

 fractor under very bad atmospheric conditions, one soon comes to 

 appreciate the enormous advantages of a high station with transpar- 

 ent air and quiet images. I will mention here one point in particu- 

 lar regarding which I have had some experience. If a telescope is 

 to be used for spectrographic investigations it is necessary to have 

 the greatest possible aperture in order to secure great light grasping 

 power. If the ordinary ratio, about i : 18, of aperture to focal length 

 is employed, this large aperture corresponds to a very great focal 

 length, and in consequence of this the images become so bad with 

 unsteady air that the advantage of the great aperture is almost 



