84 EEPORT OF THE SECRETARY. 



feet focus, fed by a new form of two-mirror coelostat, and employed with a provision 

 for "churning" tiie air on the jjath of the beam. This instrumental equipment is to 

 be used for holographic study of the solar image, and especially sun-spot energy 

 spectra and the absorption of the solar envelope. Second, in the continuation, with 

 improved holographic; apparatus, of studies of the solar-energy spectrum and the 

 absorption of radiation in the atmosphere. 



As the most notable result of the studies of atmospheric a))sori)tion, it ai)i)ears tliat 

 the average transmission of the air of Washington for all wave lengths for the best 

 days of 1902-3 has been as much as 10 per cent less than for the best days of 1901-2, 

 while the decrease of transparency in the violet is very much greater. There is no 

 evidence that this remarkable decrease is due to water vapor. Indeed, it has been 

 well observed on the very driest days. 



Inquiries have been made of the Observatory as to whether the blight of the wheat 

 and barley crops could be attributed to any decrease in the ultraviolet radiation of 

 the sun, but the Observatory was able only to state the fact that such a decrease had 

 been observed. 



It is probable that something like this affected the actinometric observations 

 recorded V>y Crova in the years following the great eruption of Krakatoa, though 

 whether the smaller one of Mont Pelee can be associated with it is uncertain. There 

 is no evidence yet obtainable of how general this absorption is at diSerent parts of 

 the globe, but it is, perhaijs, the most notable single result of our- past year's experi- 

 ments, and is eminently in the line of the work you have anticipated for the Astro- 

 physical Observatory, in furnishing data of importance to the national interests in 

 agriculture, and in estimating the influences which may affect j)ast and coming 

 harvests. 



Respec'tfully sulnnitted. 



C. G. Abhot, 

 Aid, Adbuj hi. Cluinje Ai^iro})Inisiail Observatory. 



Mr. .S. r. Lanolev, 



Secrelary of the StiiilliHonkm Iiinlitut.ion. 



