158 CARNEGIE INSTITUTION 



many problems of astrophysics the atmospheric conditions experi- 

 enced at most of our observatories are to be regarded as unfavorable. 

 Every increase in the transparency and also in the steadiness of the 

 air marks an advance, and for this reason the choice of a site for a 

 new astrophysical observatory should be made with the greatest 

 possible care. 



The best conditions of the atmosphere are to be expected at a 

 mountain station, and, according to my experience, an isolated peak 

 is to be preferred to a point within a great mountainous region, 

 where surrounding peaks under certain circumstances exercise a 

 strong influence on the state of the atmosphere. 



The higher the chosen point the better, but height is not the only 

 important factor. It is far more important that the observatory 

 shall not be too difficult of access, and, before all e\se, it is essentia 

 that the observations can be made with as great convenience audi 

 ease as at any other observatory. Occasional observations, such as 

 are made during a very short period on a very high mountain under 

 the most difficult conditions, are ordinarily of comparatively little 

 value. Many problems of astrophysics, such as the determination 

 of solar radiation and investigations on the absorption of the atmos- 

 phere, etc. , cannot be solved during short expeditions, lasting days 

 or weeks ; they demand systematic study during a long period of 

 time under the most varied atmospheric conditions — if possible, 

 simultaneously from a peak and from a valley station. 



In my opinion, it would be best to establish the proposed principal 

 observatory as high as possible, perhaps at an altitude of about 8,000 

 feet, but in any event so that it could be kept in operation through- 

 out the entire year, or at least through the greater part of it, and 

 be at all times accessible without too great difficulty. At this place 

 the principal instruments should be established, and all investigations 

 conducted which relate to the spectroscopy and the photometry of 

 the fixed stars, particularly those in which photography is employed. 

 An important requirement is the provision of a second permanent 

 station at a height of about 1,000 feet. This should likewise be 

 provided with the best instrumental equipment, with the object in 

 view to provide for certain observations which should be made simul- 

 taneously at both stations. It is naturally desirable that the direct 

 distance between the two stations should be as small as possible, and 

 that they should be connected with each other by telegraph and 

 telephone. 



As regards the program of investigations prepared by your Com- 



