132 



nutting: brightness of optical images 



of opal glass >S, 14 inches square, forming the front face of a cubic- 

 al box containing a tungsten lamp of 1500 candle power. Op- 

 posite the source, 5 meters distant, was a white magnesia block 



&-•-■::-' 



□ 



Fig. 3 



.90S 



Fig. 4 



o:s 



M upon which an image of the source could be thrown by the 

 lens L under test. The brightness of this image was compared 

 with that of the same spot with the lens removed by means of a 

 modified Beckstein illuminometer. 



