ADVISORY COMMITTEE ON AvSTRONOMY 139 



in Chicago, from February, 1892, to May, 1895, after which the in- 

 struments were removed to the Yerkes Observ^atory. On account 

 of the necessity of remodeling the Kenwood telescope for other 

 observations, this series of observations has been discontinued. 

 Many photographs of the Sun have been taken, however, with a 

 spectroheliograph attached to the 40-inch Yerkes telescope, and a 

 daily series will soon be resumed as part of an extended programme 

 of solar investigations, for which the necessary instruments are 

 nearly completed. Spectroheliographs ai-e also in ser%'ice at the 

 astrophysical observatories of Meudou and Potsdam, and a com- 

 mittee of the Royal Societ},' is about to have one constructed for 

 use in India. 



Janssen has obtained directly enlarged photographs of Sun spots 

 and photospheric grauulatious on a scale great enough to show the 

 structure admirablj'. This work is still in progress at the Meudon 

 Observatory, and some equally good results have been obtained by 

 Belopolsky at Fulkowa. It is important that further experiments 

 of thi:-, kind be undertaken at other observatories, and that a sys- 

 tematic record of the changes in spot and atmospheric structure be 

 maintained. 



It will be seen from what has been said that when all of the new 

 instruments are in use, the provision for recording the forms of solar 

 phenomena will be nearly sufficient. The photographic record of 

 Sun spots made under the direction of the Greenwich Observatory' 

 need not be duplicated elsewhere, at least in so far as the systematic 

 reduction of all the places is concerned. For comparison with spec- 

 troheliograph plates, however, direct photographs should be taken 

 at other obser\'atories. The small size of the solar image used at 

 Meudon and at Potsdam (and planned for the Indian spectrohelio- 

 graph) precludes the possibility of photographing the smaller de- 

 tails of the chromosphere. For this reason a large scale record to 

 supplement that of the Yerkes Observatorx'- should be provided for 

 at some tropical station. For studies of special phenomena an even 

 larger image could be used to advantage, and large-scale direct 

 photographs of spots should be taken for comparison. The work 

 of photographing prominences simultaneously in lines of different 

 elements, such as calcium and hydrogen, which has already yielded 

 some interesting results at the Kenwood Obser\^atory, should also 

 be continued. There are many other special investigations to be 

 made with the spectroheliograph. 



