1 8 CARNEGIE INSTITUTION 



pheric tests. The atmospheric conditions on Mount Wilson at an 

 altitude of 6,000 feet, as reported b}- Mr. Hussey, appear to be re- 

 markably fine, and this opinion is supported by Messrs. Campbell 

 and Hale, who took part in the final tests. 



Auxiliary station for solar observations. — Among the problems of 

 the proposed observatory would be that of determining what is the 

 total amount of heat radiated by the sun and to what extent, if any, 

 this amount varies — the determination of the so-called solar con- 

 stant. For this purpose it would be ideal to make this measurement 

 from a point which receives the radiation of the sun without any 

 absorption whatever by the earth's atmosphere. As an approxima- 

 tion to this, one might choose a station upon the top of the loftiest 

 mountain which can be ascended ; but for reasons already stated 

 we are not in favor of this extreme elevation. 



It has been thought that some such site as Mount Whitney, about 

 170 miles north of Mount Wilson, might offer a suitable auxiliary 

 station for the special observation of heat radiations in relation to 

 the solar constant. The altitude of this mountain is nearly 15,000 

 feet. This is an elevation beyond which it can scarcely be supposed 

 that continuous observations could be effectively carried on. In 

 1 88 1 Professor L,angley occupied this peak for a similar purpose. 

 We think it might ultimately prove that after occupation for two 

 or three seasons such a relation with the results obtained on Mount 

 Wilson might be established as to render the continued maintenance 

 of such an auxiliary station unnecessary. The principal study of 

 the heat radiations of the sun would then be made at the permanent 

 station on Mount Wilson, or wherever the main station might be 

 located. 



In the section of this report devoted to the proposed Solar Observa- 

 tory will be found a presentation of reasons for our conviction that 

 this institution should be established, together with a carefully pre- 

 pared description of the appliances that would be required. 



Advantages of a great reflector. — As connected with both the 

 Southern and the Solar Observatory, and a consistent part of them, 

 we think that a large reflector should be mounted and maintained 

 for the investigation of stellar spectra. From what we have already 

 stated, it will be seen that we regard the relation of solar physics 

 to stellar physics the most important inducement for the establish- 

 ment of the Solar Observatory, though not the only one. In order 

 to make a comparative study effective, there would be a great advan- 



