SOLAR OBSERVATORY. 1 63 



INVESTIGATIONS IN PROGRESS. 

 SOIvAR RESEARCH. 



Direct Photography of the Sun. 



Direct photographs of the sun have been made on every clear day with 

 the Snow telescope, as in previous years. 



Work with the SpEctroheliograph. 



During the year 1,180 photographs of the sun have been made with 

 the 5-foot spectroheliograph of the Snow telescope. The great advantages 

 offered by the Ha line of hydrogen for the study of the solar vortices were 

 mentioned in the last annual report. For the purposes of the daily record, 

 two photographs of the entire disk are ordinarily taken with this line and 

 when the conditions are favorable an additional series of photographs of 

 interesting regions is also made. In the study of these plates it has been 

 found that the changes in form of the Ha hydrogen flocculi are so rapid that 

 photographs separated by an interval of 8 or 10 hours can not safely be em- 

 ployed for the study of the solar rotation. With the Snow telescope, sharply 

 defined images of the sun are obtained only during short intervals in the early 

 morning and late afternoon. Consequently, it has been necessary to post- 

 pone a further investigation of the solar rotation, as determined by the mo- 

 tions of the Ha flocculi, until arrangements have been made for observations 

 of this kind with the tower telescope. 



It is evident, however, from studies of the changes in form and intensity of 

 the hydrogen flocculi, that the problem of finding objects suitable for meas- 

 urement in this research will not be a simple one. A series of 50 photographs, 

 taken at intervals of about i minute, and examined directly or with an Edison 

 kinetoscope, affords excellent material for tracing the rapid changes of the 

 flocculi near sun-spots. It may be said, in general, that distinct evidence of 

 motion along the apparent stream-lines is not usually afforded by such a series. 

 On June 3, 1908, a great mass of dark hydrogen was suddenly swept into a 

 sun-spot, and the various phases of the phenomenon were well shown by the 

 photographs. Nevertheless, it is now evident that such an event is to be 

 regarded as exceptional. In fact, it appears probable that the changes in the 

 flocculi are frequently due to variations in the temperature, depth, or density 

 of the hydrogen gas, rather than to motion of the gas along the stream-lines 

 of a vortex. Dr. Abetti devoted much of his time to work on this subject. 



In the elucidation of these phenomena, the 30-foot spectroheliograph, 

 erected in the autumn of 1908, for use with the 60-foot tower telescope, has 

 proved highly advantageous. In this instrument the collimator and camera 

 slits are fixed in position and the motion of the solar image is produced by a 

 system of three plane mirrors, mounted above the collimator slit, and moved 

 across it by means of an electric motor. A 60° liquid prism, a 64° glass 



