ON THE DISTRIBUTION OF RADIATION OVER THE 



SUN'S DISK AND NEW EVIDENCES OF THE 



SOLAR VARIABILITY 



By C. G. abbot, F. E. FOWLE, and L. B. ALDRICH 

 (With One Plate) 



Referring- to the Annals of the Astrophysical Observatory/ we 

 gave there the results of earlier investigations of the distribution of 

 radiation over the sun's disk, and some indications of a variability of 

 this distribution which might be associated with variation of the 

 sun's total radiation. This work was done in Washington prior to 

 the- year 1908. When the new observing station of the Smithsonian 

 Institution was constructed upon Mt. Wilson in 1909, provision was 

 made for the erection of a tower to be used for a tower telescope for 

 the continuation of such observations. It proved impossible to equip 

 a tower telescope until the autumn of the year 191 3, when apparatus 

 was hastily arranged and operations were begun on September 9, 

 1913. The tower was improved both in its rigidity and in the mount- 

 ings of the apparatus for the research in 1914 and 191 5, and is now 

 regularly used on all days when solar-constant observations are 

 made. 



In figure i (pi. i) is given a general view of the tower telescope 

 upon the observing station at Mt. Wilson. Owing to the bold situa- 

 tion of the station, it is impossible to get a photograph of the appara- 

 tus except by climbing a tall pine tree at some distance away, and 

 the trees intervening are some obstacle to a satisfactory presentation 

 of the installation. 



Figure 3 is a diagram of the construction of the tower telescope 

 and its relation to the spectro-bolometer. A and B are the mirrors 

 of the coelostat, from which a beam of sunlight passes downward to 

 the 30-centimeter (12-inch) concave mirror C of 23 meters (75 feet) 

 focal length. Thence the beam passes up to the plane mirror D, 

 which reflects the image to focus at E near the floor of the observing 

 chamber. At F is a small plane mirror at the angle of 45^, which 

 reflects the beam through the slit G of the spectro-bolometer G H I. 

 At / K, outside the observing chamber, is the coelostat used for the 

 ordinary solar-constant observations. 



^Vol. 2, Pt. 3, and Vol. 3, Chap. 7. 



Smithsonian Miscellaneous Collections, Vol. 66, No. 5 



