PHOTO-ELECTRIC PHOTOMETRY OF THE 1918 

 ECLIPSE. 



By JAKOB KUNZ and JOEL STEBBINS. 

 (^Read April 25, 1919.) 



The expedition from the University of Ilhnois had for the sole 

 item on its program the measurement of the total light of the corona 

 by means of photo-electric cells. The arrangement consists of a 

 light-sensitive cell connected in series with a galvanometer and a 

 battery giving about 150 volts. When exposed to a light about 

 equal to that of the crescent moon, a measurable current is pro- 

 duced, and it was anticipated that the corona would be bright enough 

 to be measured with accuracy. The advantage of this electrical 

 photometer is that no matter what the distribution of light is in the 

 source, whether is be a point or an irregular surface, the effect is 

 integrated and is combined into a single galvanometer deflection. 



The color-sensitivity of the potassium cell is nearly like that of 

 the ordinary unstained photographic plate, the maximum effect 

 being at wave-length 4500 A. In a general way, the photo-electric 

 measures are between photographic and visual, but nearer the 

 former. 



Slide I shows one of these cells, the diameter of the bulb being 

 about one inch. 



Slide 2. — The station at Rock Springs, Wyoming, was selected 

 so as to be near the Yerkes camp, where Mr. Parkhurst was to un- 

 dertake visual photometric measures of the corona, for it was felt 

 that there would be advantages in having observations by different 

 methods, but with practically the same atmospheric conditions. 

 However, we did not want a local cloud to spoil all of the pho- 

 tometry, so we located about fifteen miles east of the Green River 

 people, and about two miles south of the town of Rock Springs. 



Slide 5. — General view of the station. 



Slide 4. — It was proposed to take measures in duplicate with two 

 cells, each mounted in a box at the lower end of a simple tube. The 



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