90 president's address— section a. 



method by which the sun's surface and surroundings could be 

 regularly studied, but Professor George Hale, guided by what 

 had been done by others, studied and succeeded in working out a 

 new method, which seems to meet all, or nearly all, the require- 

 ments. He had completed this work ready to take solar photographs- 

 by January 22nd, 1892. He calls the instrument a spectro-helio- 

 graph™, and by it the solar prominences, faculaj and chromosphere, 

 can be clearly photographed by monochromatic light of the waA^e 

 length K. It is not necessary here to describe the instrument; it 

 will suffice if I mention the essential points of difference i>etween 

 the spectro- heliograph and an ordinary solar spectroscope. Let us- 

 suj)pose, then, that we have a solar spectroscope. The professor 

 removes the fixed slit, and puts in its place one large enough to 

 take in the whole of the sun and surroundings ; this slit can, by 

 suitable machinery, be made to move across the image of the sun. 

 The^'grating is next so adjiisted as to give only the K line of the 

 spectrum. The ordinary eyepiece for viewing the spectrum is next 

 removed and in its place is fixed another movable slit, which is- 

 moved by the same machine as the other one, and at a definite- 

 relative rate ; all being adjusted, the light of the K line of the- 

 spectrum will pass from the grating through the second slit, the 

 use of which is to prevent any side light near the K line from 

 falling on the sensitiA-e surface ; to complete the arrangement it is 

 only necessary to put a sensitive plate very close to the second slit. 

 Everything being ready the telescope is uncovered and the slits set 

 in motion. As the first moves across the image of the sun the- 

 second moves across the sensitive plate, and any K light passes 

 through it and leaves its record on the plate in a position 

 relative to that on the sun. Thus, practically, a series of very fine 

 lines, sections as it were, a(^ross the prominences are recorded side 

 by side until the whole disc is included, and the photosphere and. 

 prominences clearly photographed. The operation requires first- 

 rate apparatus ana every precaution to ensure success 1 will only 

 mention one contrivance. When the slits are adjusted and everything 

 ready to take the photograph, a round disc of metal is put in front 

 of number oire slit ; it is nearly as large as the image of the sun, 

 and practically makes an eclipse of the brighter parts, leaving only 

 the edge of the sun, the photosphere, and ])rominence-i and corona 

 to pass to the spectroscope." The next point is to secure on the 

 same sensitive plate a photograph of the faculaj and spots. The 

 grating is now set, so that the combined slits only allow the 

 faculaj light to pass. All is then prepared, so that the slits will 

 move across tlie sun's disc and across the plate. The disc is re- 

 moved, and the faculaj >pots recorded in their true relative 

 positions to the prominences. The apparatus is quite successful, 

 and the professor thinks that, with a modification, he can alsO' 

 photograph the corona ; but up to latest reports this has not been 

 successful. 



