248 CARNEGIE INSTITUTION OF WASHINGTON. 



out. This is so gradual that no appreciable change in the admirable 

 constancy of temperature has been caused, while the exposure times 

 have been reduced practically one-half by the removal of the moisture. 



DIRECT PHOTOGRAPHY OF THE SUN. 



During the year ending August 31, 1914, 241 direct photographs of 

 the sun were obtained with the Snow telescope. Few spots were 

 recorded in the autumn and winter, but the advent of a new cycle of 

 activity is now plainly apparent. 



WORK WITH THE 5-FOOT SPECTROHELIOGRAPH. 



952 photographs have been made with the 5-foot spectroheliograph, 

 using Ha and K2 for the disk and for the prominences at the limb. As 

 usual, these plates have furnished material for the measurement of the 

 areas of the calcium flocculi and prominences by Miss Smith, and for 

 various studies of spots and flocculi in connection with other phases of 

 solar research. 



GENERAL MAGNETIC FIELD OF THE SUN. 



Our knowledge of the general magnetic field of the sun is based upon 

 line displacements so minute as to fall well within the ordinary limits 

 of error of spectroscopic measurements. A very conservative attitude 

 is thus demanded, and no effort should be spared to verify the reality 

 of the effect, which sometimes fails to appear in the measures of skilled 

 observers. Most of the measures used in our investigations have been 

 made by Mr. van Maanen with a parallel plate micrometer, but three 

 other observers have also detected the opposite sign of the displace- 

 ments in the northern and southern hemispheres with a measuring 

 machine of the ordinary type. Mr. Capon, using a parallel plate 

 micrometer, has also obtained results in good agreement with those of 

 Mr. van Maanen. Finally, a series of measures on curves made by 

 Professor Koch with his registering micro-photometer, which eliminates 

 all possibility of personal equation, agree closely with Mr. van Maanen's 

 measures of the same plates. The large Koch machine just completed 

 in our instrument shop will serve for further work of this nature, and 

 especially for the elimination of absolute errors due to personal equation. 



During the year 2,178 photographs of spectra for the study of the 

 sun's general field have been taken with the 150-foot tower telescope 

 and 75-foot spectrograph by Mr. Ellerman and Mr. Monk. The earlier 

 photographs, many of which were made at irregular intervals because 

 of broken weather and other interruptions, were designed mainly for 

 the detection of new lines showing Zeeman displacements, the deter- 

 mination of field strengths for lines representing different levels in the 

 solar atmosphere, and the selection of lines suitable for use in a cam- 

 paign planned for the exact location of the sun's magnetic poles and the 



