234 CARNEGIE INSTITUTION OF WASHINGTON. 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 



INSTRUMENTS. 



The exceptional qualities of the photographs of flocculi obtained 

 with the 13-foot spectroheliograph, built for the 60-foot tower telescope 

 a year ago, have thrown heavy demands upon these instruments, 

 which are used daily throughout the period of best seeing. In order to 

 take full advantage of the good definition afforded by the 60-foot 

 tower, and to set free the Snow telescope for other work, the 5-foot 

 spectroheliograph was mounted horizontally in the house at the base 

 of the tower in December, together with a 45° plane mirror for project- 

 ing the solar image upon the vertical slit of the spectroheliograph. 

 This mirror can be moved to one side, pennitting the solar image to be 

 used with the 13-foot spectroheliograph or the 30-foot spectrograph, 

 w^hich hang side by side in the concrete chamber beneath the tower. 



The Snow telescope has two advantages over the tower telescope: 

 its perfect achromatism (due to the exclusive use of mirrors in its 

 optical train), and its comparatively large angular aperture, which 

 is twice as great as that of the 60-foot tower telescope. On the other 

 hand, the distortion of the large and relatively thin mirrors, when 

 exposed to sunlight, limits seriously the period of good definition, and 

 frequently produces a very appreciable change in the focus of the 

 solar image during a single exposure with the spectroheliograph. This 

 telescope is, therefore, best adapted for investigations in which large 

 angular aperture and perfect achromatism are more urgently needed 

 than perfection of focus and good definition. 



We have therefore decided to equip the Snow telescope with a vertical 

 spectrograph, mounted in an underground chamber of concrete, of 

 the type which has proved so satisfactory in our previous work with 

 the two towers. The combination will be available for Mr. St. 

 John's investigations on the relative positions of arc and solar lines, 

 which have been handicapped by the small angular aperture of the 

 tower telescopes. It is extremely important to have the diameter of 

 the hght cone at the grating level much larger than the aperture of the 

 grating itself, in order to assure perfect unifonnity of illmnination. 

 This new apparatus will soon be ready for use. 



SOLAR PHOTOGRAPHY. 



During the year ending August 31, 1916, 84 direct photographs of 

 the Sun were taken with the Snow telescope prior to the removal of the 

 5-foot spectroheliograph, together with 222 spectroheliograms, includ- 

 ing 130 Ha and 46 K2 images of the entire disk, and 46 K2 promi- 

 nence plates. 



