MOUNT WILSON SOLAR OBSERVATORY. 1 73 



The whole investigation has been laborious and tedious, and the final re- 

 sult, so far as the satisfactory determination of photographic magnitudes is 

 concerned, is still somewhat problematical. There is reason, however, for 

 believing that the various difficulties may be overcome. 



Stellar Spectroscopy. 



The three-prism spectrograph for use at the 8o-foot focus of the large 

 reflector, described briefly above, has been used for almost all of the stellar 

 spectroscopic work for the year. A few photographs have been obtained 

 with the small focal-plane instrument, but it has seemed desirable to delay 

 any extensive series of observations with it until the new steel frame already 

 referred to should be available. The considerable gain in light efficiency 

 with this form of mounting should prove of especial advantage for work 

 upon faint stars. 



The program of stellar spectroscopic work with the Cassegrain spectro- 

 graph at the present time includes determinations of the radial velocities of 

 two lists of stars : first, about 400 stars, mainly of types A and B, selected by 

 Professor Kapteyn for the evidence which they will furnish concerning star- 

 streams among the stars with these types of spectra ; second, about 90 stars 

 between magnitudes 5 and 7, mainly of the solar type of spectrum, for 

 which the parallaxes and the proper-motions have been determined. In ad- 

 dition, photographs are being made of a small number of stars of type A 

 with the object of making accurate studies of the character of their spectra. 

 The Observatory will also take part in the general cooperative plan now 

 under consideration at several observatories for the determination of the 

 radial velocities of all stars between magnitudes 5.0 and 6.5. 



Since December about 600 photographs of the spectra of nearly 200 stars 

 have been obtained with the Cassegrain spectrographs by Messrs. Adams, 

 Babcock, and Pease. Experience has shown that in the case of stars having 

 spectra with few and, as a rule, ill-defined lines no gain in accuracy is ob- 

 tained by increasing the linear scale of the photographs beyond a certain 

 amount, while in extreme cases a distinct loss is suffered. After some ex- 

 periments in this direction we have adopted provisionally a dispersion of two 

 prisms used in conjunction with a camera of 18 inches focal length as most 

 suitable for the study of the stars of types A and B and the fainter solar 

 stars as well. Since the collimating lens in this instrument has a focal length 

 of 40 inches, the combined optical train is very efficient as regards the purity 

 of the resultant spectrum, a consideration of especial importance in photo- 

 graphing stars of the solar type of spectrum. The efficiency of the spectro- 

 graph when used in this form has been very satisfactory and fully up to our 

 anticipations, in spite of the fact that the light undergoes four reflections at 

 silver surfaces before reaching the photographic plate. Under good condi- 

 tions fully exposed negatives of stars having a visual magnitude of 6.0 are 

 obtained in an hour. The faintest star observed up to this time has a magni- 

 tude of 7.2 on the visual scale. 



