62 REPORTS ON INVESTIGATIONS AND PROJECTS. 



DIRECT PHOTOGRAPHY OF THE SUN (Messrs. Hale and Euerman). 



Photographs of the sun on a scale of 6.7 inches to the solar diam- 

 eter are taken daily with the Snow telescope. With the present 

 exposing shutter the aperture of the 60-foot mirror must be reduced 

 to about 3 inches when Process plates are used. A separate photo- 

 heliograph will be provided later, so that the Snow telescope may be 

 used exclusively for work with the spectroheliograph during the 

 hours of best definition. Many of the direct photographs already 

 obtained are very sharp and have been of great service in comparisons 

 (with the stereocomparator) of faculae with H, photographs of the 

 flocculi. 



WORK WITH THE SPECTROHEUOGRAPH (Messrs. Hale and 



Ellerman). 



The temporary spectroheliograph, built for use with the Snow tel- 

 escope pending the construction of the 5-foot spectroheliograph, is a 

 simple but very efficient instrument. A heavy wooden base (saturated 

 with melted paraffin to prevent change of shape in moist and dry- 

 weather) carries two iron rails of V section. Four steel balls run in 

 these V's and in the inverted V's of two iron rails on the lower side 

 of the wooden platform that bears the slits and optical parts. The 

 first slit, the collimator, and the camera are from the old Kenwood 

 spectroheliograph. The two prisms, of 64 refracting angle, were 

 made for the Bruce spectrograph of the Yerkes Observatory, but had 

 to be replaced in that instrument because of their imperfect definition. 

 This is sufficiently good, however, for spectroheliographic work, 

 where the requirements in this particular are less severe than in 

 stellar spectroscopy. In the absence of a suitable mirror, to give the 

 required deviation of 180 , the silvered face of a third prism is em- 

 ployed. The second slit and plate-holder support belonged to a spec- 

 troheliograph of the type suggested by Newall, which was built for 

 experimental purposes at the Yerkes Observatory. The solar image 

 and plate are fixed and the spectroheliograph is moved across them 

 by means of a small electric motor. This is belted to a large wooden 

 pulley on a screw supported by the wooden base and running in a 

 nut attached to the moving platform. 



The optical parts in this simple instrument are small, and there- 

 fore only a narrow zone of the 6. 7 -inch solar image can be pho- 

 tographed. For special studies of spot regions and of interesting 

 flocculi, however, no better instrument could be desired. The great 

 advantages of a spectroheliograph mounted on a pier over one attached 

 to an equatorial telescope have made themselves evident from the 



