STANDARDIZATION AND EFFICIENCY OF THE FOG CHAMBER. 



37 



29. Data for apertures. To identify the coronas in case of the small 

 vessel, No. 3, as to the order to which they belong, a special goniometer 

 was constructed. In using this the eye is placed at the nearer wall of the 

 fog chamber, while the two round vertical rods for defining the angular 

 aperture are placed beyond the further wall, on a radius of 30 cm., the 

 eye being the center. The inner edges of the rods determine the aperture 

 by aid of a sliding scale passing across them. In this way very large 

 coronas may be measured in relatively small apparatus. The angular 



-The same as fig. 10, obtained by goniometer measurement, including Wil- 

 son's colors and the effect of weak radium. 



