MOUNT WILSON SOLAR OBSERVATORY. •..- 155 



work has been carried on by Miss Smith with the aid of the special photo- 

 meter devised for this purpose and referred to in the report of last year. The 

 method of reduction which has been followed is the same as that previously 

 described and the results obtained have proved to be fully as satisfactory as 

 the character of the objects measured would seem to warrant. The values 

 obtained for a period of about seven months have been platted and indicate 

 very clearly the variation in the areas covered by the flocculi as the spot- 

 groups pass across the surface of the sun. The time interval is of course too 

 short to give any indication of the eleven-year period, but there is every rea- 

 son to suppose that these determinations will furnish an excellent measure of 

 the sun's activity. 



The measurement of the plates used in the determination of the rotation of 

 the sun has been done for the most part by Miss Lasby, and in addition she 

 has devoted considerable time to the measurement of the displacements found 

 in the spectra of the center and limb of the sun. The extension of the rota- 

 tion work to include the lines of hydrogen and the line of calcium at A 4227 

 has increased considerably the amount of measurement which it has been nec- 

 essary to do in carrying out this research. 



The definitive reduction of our plates of the sun-spot spectrum, and the 

 identification of the unknown lines which appear on them, has involved a 

 great amount of measurement and reduction. This work has been carried on 

 for the most part by Miss Burwell and Miss Wickham. Miss Burwell has 

 determined the wave-lengths of the sun-spot lines, and Miss Wickham has 

 measured, for purposes of identification, plates of the titanium-oxide flutings 

 obtained in the laboratory. Some idea of the amount of labor involved may 

 be obtained from the fact that in the extent of spectrum between A 5000 and 

 A 5500 there occur over 1,500 lines, whose wave-lengths in the spot spectrum 

 must be determined and for which suitable identifications with laboratory 

 spectra have to be found. 



In addition to her work on the measurement of the titanium-oxide flutings, 

 Miss Wickham has devoted much time to the estimation of the intensities of 

 the lines upon the plates of furnace spectra obtained by Dr. King. 



CONSTRUCTION DIVISION. 



The construction division, under the superintendence of Professor Ritchey, 

 has been occupied during the year with the erection on Mount Wilson of the 

 steel building and dome for the 60-inch reflector ; the completion of this in- 

 strument, its transportation to the mountain and its erection in the dome ; the 

 erection and equipment of the new spectroscopic laboratory in Pasadena ; the 

 construction of a spectroheliograph of 30 feet focal length and a grinding 

 machine for the 100-inch Hooker mirror ; the completion of the Mount Wil- 

 son road ; and other miscellaneous work. 



