REPOKT OP THE SECRETARY 105 



the reduction of observations made at Mount Wilson in 1922 on the 

 form of the solar spectrum energy curve and on the spectrum energy 

 curves of ten of the brighter stars. 



It was apparent from the comparison just referred to between 

 the results of the two field stations that they were in close accord on 

 the sun's variation. But the Chile station was employing in its 

 reductions the results of work prior to 1913 on the distribution of 

 radiation in the solar energy spectrum; while the Arizona station 

 was employing results of 1920. Moreover, the pyranometer in use 

 in Chile was of an old type unsuitable to the work. Furthermore, the 

 sharpness of definition of the spectrum employed at Mount Harqua 

 Hala was inferior to that employed at Montezuma. It seemed prob- 

 able that to remedy these defects and put the two stations on equality 

 in all respects would lead to even closer harmony in their results, 

 although it meant a revision of the whole scheme of reductions at 

 both stations, with a redetermination of the systematic errors at each. 

 This is very important, for the solar variations rarely exceed 5 per 

 cent, and are mainly less than 2 per cent. It taxes the best observing 

 to reveal them. These were the circumstances which led to the large 

 computing program stated in the preceding paragraph. It has re- 

 sulted in putting the two stations on equal footing in every possible 

 way. They are now capable of turning out jointly the best results 

 on the solar variation that our experience can suggest a means to 

 attain. 



The instrument maker, Mr. Kramer, has been on detached service 

 for almost the entire year, engaged in the preparation, according 

 to plans of the director, of two solar radiation outfits ordered, 

 respectively, by a committee of interested gentlemen in Australia 

 and by the Government of Argentina. The Australian outfit was 

 finished and sent forward in June, 1923. The Argentine outfit will 

 go forward about December, 1923. 



Field work at Mount Wilson. — Messrs. Abbot and Aldrich ob- 

 served on Mount Wilson in the months of July, August, and the 

 fore part of September, 1922. They redetermined the form of the 

 solar spectrum energy curve. For this investigation they employed 

 several different prisms, including two of rock salt. Their object in 

 this course was to vary the procedure, as far as possible, so as to 

 get several independent checks on the results. Upon reduction, all 

 of the results of 1922 came into good accord with one another, and 

 confirmed the work of 1920 very satisfactorily. It now appears 

 that a large part of the earlier work, on which results published in 

 Volumes III and IV of the Annals were based, was injured in ac- 

 curacy by the employment of a quartz prism of inferior transparency. 

 If this quartz prism work is rejected, the remaining early work is 



