Polarization Flicker Photometer, 



363 



two glasses. If: used with the polarization apparatus the 

 divergence of the double-image prism-beams easily clears 

 the gap between glasses. A thin metal screen is placed in 

 this gap to prevent illumination of one glass by the other, 

 and the edges of the glasses are coated with opaque black 

 paint. 



Fig. 3. 



Mixture photometer : O x and 2 are opal glasses turned in opposite 

 directions by the milled head M. 



With the glasses turned at right angles to each other and 

 normal to the two photometer tracks the two glasses form the 

 two halves of an ordinary equality photometer field. When 

 turned at other angles various mixtures of the two lights are 

 obtained. The brightness of each glass at various angles 

 due to illumination by each light alone was carefully deter- 

 mined by measurements with a portable photometer. In 



2 B2 



