314 NUTTING AND JONES: PHOTOMETER FOR SMALL AREAS 



2. The elimination of corrections for shift of zero by providing easy 

 means of balancing the two beams with an open sj^stem. 



3. A direct view of the object sighted upon at all times dm-ing 

 measm-ement. This is obtained by focusing an image of that object 

 at the dividing line of the photometer cube. 



4. An open linear scale, reading directly from zero to 100 per cent 

 provided by a rotating comparison beam and stationary sector. The 

 photometer head used is, in fact, that of the Bechstein illuminometer. 

 The ease and rapidity of setting provided by this means of varying the 

 (Comparison beam are known to all who have used it. 



A plan of the optical parts of the photometer is shown in the 

 figure as used for determining transmissions. 



f--i 



.L, 



R2 



V 



Fig. 1. 



The source of light £' is a condensed filament, 100-watt tung- 

 sten lamp run on the lighting circuit. Light from this illumi- 

 nates the comparison screen c of thin, solid opal glass. This 

 screen is viewed through the ocular 0, the photometer cube P, 

 the rotating excentric lens L2, and stationary sector >S. Light 

 from E also passes to the photometer cube through the path 

 ER1R2P. Ri is a reflecting prism within a metal box mounted 

 on a stand. The front face of this box is a plate of flashed opal 

 glass D, serving as a secondary source by diffusing the ight. 

 >The plate whose transmission is to be determined is placed 

 directly in front of this diffusing screen, thus eliminating errors 

 due to diffusion in the transmission measured. An image of 

 the plate to be observed is thrown on the photometer cube P, 

 by means of the lens Li, and the reflecting prism R2. These 



