6o REPORTS ON INVESTIGATIONS AND PROJECTS. 



would be seriously handicapped by these changes of focal length, 

 n^t only because of the poor definition caused by astigmatism, but 

 also because the long exposures required with the spectroheliograph 

 would be impracticable under such conditions. Attention was there- 

 fore concentrated on two objects: (i) the provision of a remedy 

 for the difficulty experienced with the mirrors ; (2 ) the reduction of 

 exposure time in the spectroheliograph. 



The solar definition at Mount Wilson is best about an hour after 

 sunrise. At this time the mirrors, after being cooled by the night 

 air, might be supposed to give most trouble on account of the rapid 

 change of temperature after sunrise. It was accordingly thought 

 that it might be desirable to maintain the mirrors during the night 

 at a temperature approximating that attained during the day after 

 exposure to the sun. Experiment soon led, however, to a different 

 solution of the problem. At the time of best definition the radiation 

 from the low sun undergoes considerable absorption in the earth's 

 atmosphere. Although this makes necessary an increased exposure 

 in the spectroheliograph, it is nevertheless very advantageous in pre- 

 venting rapid heating of the mirrors. In fact, if the exposure time 

 is made as short as possible and the mirrors shielded from the sun 

 between exposures by an adjustable canvas screen, the change of 

 focus can be so reduced as to become of small importance during the 

 period of best definition, which lasts about an hour. 



The experiment has been tried of cooling the mirrors with a blast 

 of air after they have been exposed for some time to the sun. The 

 good effect of this treatment was shown b}^ a considerable change of 

 focus toward the normal position. Electric fans are accordingly 

 being installed to blow each of the three mirrors while they are 

 exposed to the sun during the adjustment of the spectroheliograph, 

 and between exposures, when the mirrors are also shielded by a can- 

 vas screen. The success attained in decreasing the exposure time 

 with the spectroheliograph is described below. The result of the 

 combined efforts was to yield photographs of the sun equal, if not 

 superior, to the best obtained with the 40-inch Yerkes telescope and 

 the Rumford spectroheliograph. When it is remembered that the 

 spectroheliograph employed in the present work is an instrument of 

 the simplest construction, extemporized for use until the permanent 

 spectroheliographs can be completed, it will be seen that such results 

 are to be regarded as satisfactory. 



It should be added that photographs of the sun suitable for most 

 classes of work can be made with the Snow telescope at almost any 

 hour of the day. Advantage is taken, however, of the fine definition 



