MOUNT WILSON SOLAR OBSERVATORY. 203 



difficulty. This is true even when the objective of 150 feet focal 

 length is employed. It is a satisfaction to record the final solution 

 of a difficulty which threatened to be serious. 



The great number of plates required for the study of the general 

 magnetic field of the sun has made it necessary to devote most of 

 the time of the 150-foot tower telescope to this subject, and for this 

 reason comparatively little has been done with the spectroheliograph. 

 Monochromatic images of the sun have nevertheless been obtained 

 with both high and low dispersion, and the results are so satisfactory 

 that much use will be made of this spectroheliograph as soon as the 

 solar activity increases. 



The heliomicrometer has been moved from its former position at 

 the rear of the shop building to its permanent place in the basement 

 of the new office building. Installed on a massive pier, under the 

 most favorable conditions for use, this instrument is available for a 

 variety of work. 



The remarkably successful application of Professor Koch's regis- 

 tering microphotometer to the determination of the positions and 

 intensity curves of solar and stellar lines will have an important 

 bearing on future research. The positions of symmetrical lines on 

 photographs of spectra can be measured with this instrument to about 

 1 micron, and since increase in their width involves no greater diffi- 

 culty in measurement, the precision becomes directly proportional to 

 the linear dispersion and hence to the focal length of the spectrograph. 

 Thus the advantages in solar research of such an instrument as the 75- 

 foot spectrograph, which have already appeared in the detection of 

 the sun's magnetic field, will be multiphed several fold by this powerful 

 auxiUary. 



Direct Photography of the Sun. 



During the year ending August 31, 1913, 222 direct photographs 

 of the sun were obtained with the Snow telescope. On account of 

 the low state of the solar activity, the very few spots recorded have 

 been small and unimportant. 



Work avith the 5-foot Spectroheliograph. 



During the year 813 photographs of the sun have been made on 

 297 days with the 5-foot spectroheliograph, using Ha and K2 for the 

 disk and Ha for the prominences at the limb. Since Februarj^ 17, 

 1913, photographs of the prominences have been made with both 

 Ha and K2, in order to provide material for a comparative study of 

 the distribution of hydrogen and calcium. 



The measurement of the areas of the calcium flocculi and prom- 

 inences has been continued by Miss Smith, whose reductions are 

 complete to August 1, 1913. Miss Smith has also made 502 prints of 

 spectroheliograms, which have been bound for reference in continu- 

 ation of the previous series. 



