178 REPORTS ON INVESTIGATIONS AND PROJECTS. 



tion of the solar image is much better than that of the Snow telescope after 

 the early morning hours. There is no convection up the vertical tube, and 

 the protection of the beam from the warm air rising from the heated earth 

 is doubtless the cause of the improvement. 



The 75-foot spectrograph has also proved to be very satisfactory. With 

 the Michelson grating the definition of the solar spectrum is excellent in the 

 first three orders ; in the third spectrum, where the distance between the D 

 lines is over an inch, both D^ and Dj are clearly double. A large number 

 of excellent photographs of spectra have been taken with the focal length of 

 75 feet (22.88 m.), and for some work the spectrograph has also been used 

 with a focal length of 30 feet (9.15 m.), since the exposure times are of 

 course much shorter in this case. The ease with which the base of the in- 

 strument can be shifted from the bottom of the well to the 30-foot level is 

 a valuable feature of the design. 



Another advantage of this instrument is the possibility of changing the 

 dispersion through a very wide range without disturbing the adjustments. 

 For example, the following series of photographs of a sun-spot group might 

 be taken in rapid succession : 



( 1 ) Structure of the group, using the instrument as a spectroheliograph, 

 with the camera-slit set on the continuous spectrum. 



(2) Calcium (Hj or Kg) flocculi, integrating the phenomena of different 

 levels. 



(3) Hydrogen (Ha) flocculi, showing the vortex structure. 



A table at the base of the instrument is then rotated, bringing a 60° prism 

 or a grating into position. 



(4) High-level hydrogen flocculi, using center of Ha. 



(5) High-level (H3 or Kg) calcium flocculi. 



If Ha indicates radial motion, a double mirror is rotated into position 

 beneath the prism, replacing the single mirror used before. This gives two 

 images of the spectrum from the same slit. 



(6) Ascending and descending hydrogen flocculi (two camera-slits, one 

 set on violet edge of Ha, the other on red edge). 



Additional spectroheliograph images, with higher dispersion or with other 

 lines, may be taken if desired. The instrument is then used (with a grat- 

 ing) as a spectrograph, for the following additional record : 



(7) Magnetic survey of the spot group, showing polarity and strength of 

 the field in different regions and at different levels. Single exposure with 

 multiple slit and polarizing apparatus. 



(8) Radial motion of spot-vapors, using multiple slit without polarizing 

 apparatus. 



(9) Spectrum of the umbra with high dispersion, for identification of 

 spot-lines, determinations of pressure, strength of magnetic field at different 

 levels, etc. 



In such a program, the order in which the photographs are taken and the 

 size of the solar image employed will naturally depend upon the special re- 



