CHARTING THE UNIVERSE 



fessor Male's spectroheliograph, with which wonder- 

 ful photographs are made that reveal the location in 

 different layers of the sun's atmosphere of hydrogen 

 gas, calcium vapor, and other chemicals. 



AIDS TO VISION SUPPLIED BY PHOTOGRAPHY 



The photographic plate had proved in recent years 

 almost as important an accessory as the spectroscope. 

 It reveals and charts with accuracy myriads of invisi- 

 ble stars. It co-operates with the spectroscope by 

 making a permanent record of the tell-tale lines 

 which that instrument dissects out of a beam of light. 



So important, indeed, is the photographic plate, 

 that it largely takes the place of the human eye in 

 direct observation of the heavens. Prof. W. W. 

 Campbell says that a six-inch telescope and a photo- 

 graphic plate will measure the velocities of stars that 

 could not be tested by the eye alone with the great 

 36-inch lens of the Lick Observatory. The 32-inch 

 lens of the Potsdam Observatory is designed exclu- 

 sively for photographic work, and not for direct 

 vision. No lens brings all the rays of light quite to 

 the same focus, and the rays that are best for photo- 

 graphic purposes are not those best for direct vision. 

 So a selection must be made. This does not apply, 

 however, to the reflecting telescope, the mirror of 

 which may be made to bring all the rays to an accu- 

 rate focus. 



The task of taking photographs with these large 

 telescopes is an exceedingly delicate one. Sometimes 

 the negative must be exposed for many hours or even 

 on successive nights, and it is necessary; to keep 



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