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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



leaves a hole of 32 ° plane angle open to the sky. The screen is brightly 

 polished on the outside, but blackened on the inside, in order to avoid 

 multiple reflections. 



The instrument to which this arrangement was attached was 

 pointed to different parts of the sky, and the zenith angle was read 

 in a circular scale, as is shown in figure 8. The value of the radiation 

 within the solid angle csd (32 ) was obtained in the usual way 



----.<* 



Fig. 8. — Apparatus used for determining the radiation to 

 different parts of the sky. 



by determining the compensation current through the black strip. 

 This arrangement has two obvious advantages over a bolometer 

 arranged in a similar way. In the first place, the instrument is very 

 steady and quite independent of air current, because both strips are 

 here exposed in exactly the same way. The readings must further be 

 quite independent of the position of the strips, it being possible to 

 turn the instrument over in different directions without change in 

 the sensitiveness. Everyone who is familiar with bolometric work 

 knows the difficulty that sometimes arises from the fact that the 



