SOLAR obse;rvatory. 147 



area covered by the black paint, in each 10° square, is then measured with 

 the photometer. 



While it might appear that the selection of flocculi must be very arbitrary, 

 and subject to large errors, it has not yet been found possible to devise a 

 more satisfactory method. Miss Smith, who has carried on this work from 

 the beginning, has acquired considerable skill in the selection of objects to be 

 measured, and the same plate, when independently measured on different 

 dates, gives results as accordant as could be expected under the circumstances. 



During a recent visit to England I discussed this subject with Professor 

 Turner, who had devoted much attention, when chief assistant at Greenwich, 

 to the similar problem of measuring the areas of the faculse, and with Mr. 

 Maunder, who now has charge of the measurement of the direct solar photo- 

 graphs of the Greenwich series. They agreed that the selection of faculse 

 on the Greenv/ich plates is necessarily an arbitrary matter, so that precise 

 determinations of their areas can not be obtained. They also considered that 

 the areas of the flocculi, as measured here, are as precise as the nature of 

 these objects warrants. If possible, the method of measurement will be 

 improved, but it appears extremely doubtful whether better results can 

 readily be obtained. 



Miss Lasby has devoted most of her time to the measurement of photo- 

 graphs taken by Mr. Adams for the spectrographic determination of the 

 solar rotation. The results of this investigation, which are given elsewhere 

 in this report, indicate that a very high degree of precision has been obtained 

 in the measures. Plates will be taken in the future, from time to time, in 

 the hope of detecting any possible variation in the rotation period. Miss 

 Lasby has also devoted some time to the measurement of photographs of 

 spectra corresponding to the center of the sun and to points near the limb. 



Miss Burwell, who has only recently joined the computing division, has 

 been engaged for the most part in determining the scale errors of the Hart- 

 mann spectro-comparator recently received from Zeiss, and of the smaller 

 Gaertner measuring machines. 



In addition to the time devoted to the general supervision of the work, 

 Mr. Adams has made a large number of measures of spectra. He has also 

 taken many photographs on Mount Wilson and has given much attention to 

 the reduction and discussion of observations. 



CONSTRUCTION DIVISION. 



The work of the construction division, which has been continued under the 

 superintendence of Mr. Ritchey, has been much heavier than in any previous 

 year. The various operations in progress have included the work of the 

 optical and instrument shops, the completion of the Mount Wilson road, the 

 transportation of materials to the summit of the mountain, the construction 

 of the Hooker building in Pasadena, and the erection of buildings on Mount 

 Wilson. 



