202 CARNEGIE INSTITUTION OF WASHINGTON. 



related problems. Professor Carl Stormer, of the University of Christi- 

 ania, Research Associate during 1912, has completed his theoretical 

 investigation of unipolar hydrogen flocculi. Professor Peter Paul 

 Koch, of the University of Munich, who arrived in Pasadena early 

 in August, is investigating the applications of his registering micro- 

 ])hotometer to the measurement of solar, stellar, and laboratory 

 spectra. Dr. J. A. Anderson, of the Johns Hopkins University, has 

 been engaged during the 3'ear on the design and construction of a 

 machine for ruling diffraction gratings. Mr. P. J. van Rhyn has 

 remained with the Observatory as volunteer assistant and has 

 devoted his time to the study of the color indices of selected stars 

 and to other problems relating to the general question of the absorp- 

 tion of light in space. 



Mr. L. B. Aldrich, of the Smithsonian Institution, has continued 

 the regular solar-constant work of the Smithsonian Astrophysical 

 Observatory during the absence of Director C. G. Abbot, who 

 returned to Mount Wilson September 1. 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 



Instruments. 



Several important additions and accessories have been provided 

 for the 150-foot tower telescope and the 75-foot spectrograph. These 

 include the permanent mountings for the 30-foot and 60-foot objec- 

 tives of the tower; a permanent water-circulation system for con- 

 trolling the temperature of the coelostat and second mirrors, to replace 

 the temporary arrangement used experimentally last year; and certain 

 minor accessories. The temporary wooden house at the summit of 

 the tower has also been replaced by one of metal, which is larger and 

 more commodious, and provides perfect protection for the instru- 

 ments in the most severe winter storms. 



Our favorable opinion of the 150-foot tower telescope, based upon 

 preliminary observations, has been confirmed by the experience of 

 the past year. The vertical tube, which is provided with a double 

 sheathing of metal and an outer sheathing of canvas, with intervening 

 air-spaces, is so completely protected against heating that no difficulty 

 whatsoever is experienced from convection currents. This is clearl}^ 

 shown by the behavior of dust particles floating in the tube above the 

 spectrograph and illuminated by the beam of sunlight. These are 

 as quiet and undisturbed as in the air of a tightly closed room. 



The trouble at first experienced from distortion of the mirrors 

 by heating of the edges has been so completely surmounted by the 

 circulation of water through the cells surrounding the glass that all 

 astigmatism has disappeared, and the change of focus has been 

 reduced to so small an amount that it is no longer a source of any 



