ASTRONOMY — HALS. 73 



the spectroheliograph. As already stated, however, much more work 

 along these lines must be undertaken, and special apparatus has accord- 

 ingly been prepared for this purpose. 



Spectrographic Investigations of the Solar Rotation. 

 The exceptional opportunity for a photographic investigation of the 

 rotation of the sun afforded by the Snow telescope has been recognized 

 from the outset, since the two essentials for securing results of the 

 highest precision, namely, powerful spectroscopic apparatus and a large 

 image of the sun, are both available. After some consideration it seemed 

 desirable to employ an apparatus of the type first used by Langley for 

 bringing the opposite edges of the sun side by side on the slit of the 

 spectrograph, and in the spring of this year an attachment of this kind 

 for the large Littrow spectrograph was completed. 



A number of plates have been secured with this instrument, by Mr. 

 Adams, and it has proved most satisfactory in its working. The excel- 

 lent quality of the grating hitherto employed (the Kenwood 4-inch) has 

 made it possible to use the fourth order of the spectrum, and so to obtain 

 the benefit of great linear scale and resolving power without impairment 

 of definition. Considerable attention has been given to methods for de- 

 riving heliocentric coordinates, and also to the question of the elimination 

 of errors arising from change of focal length of the telescope during a 

 series of exposures. 



Preliminary measures made by Mr. Adams, on a few plates, indicate 

 that the probable error for a single line is slightly less than that obtained 

 by Halm for his series of visual observations. The possibility of com- 

 bining the results for a considerable number of lines should, accordingly, 

 result in a decided gain in the degree of accuracy. The measures so far 

 made also suggest some interesting differences in the values given by 

 the lines of different elements, but the work is as yet not sufficiently 

 advanced to speak with certainty on this point. 



BoLOGRAPHic Investigations. 

 Bolographic observations with the Snow telescope were first made by 

 Mr. C. G. Abbot in the summer of 1905. The pressure of the work in 

 his observational program, however, prevented him from giving much 

 time to these experiments, and I accordingly planned to continue them m 

 connection with our solar investigations. We are indebted to Mr. Abbot, 

 not only for much valuable advice as to the arrangement and use of the 

 bolographic apparatus, but also for the loan of instruments without which 

 the work could not have been undertaken this summer. The daily pro- 

 gram of observations, which has been carried out by Mr. Palmer, has 



