42-4 Natalie C. B. Allen and T. H. Lahy: 



Time of Development. 



Di 



^z 





J>f 



3 minutes 



1.83 



2.49 



- 



.37 



4 minutes 



2.52 



3.42 



- 



.45 



5 minutes 



2.57 



3.56 



- 



.58 



Evidently, after 4 minutes' development, the density due to any 

 exposure does not appreciably increase, though prolonged develop- 

 ment does increase the fog-density. 



The transparency of a plate is defined as the ratio of the inten- 

 sity of the transmitted light to that of the incident light, i.e. 



T = — The opacity, 0, is the reciprocal of the transparency, and 



the density, D, is defined thus : — 



D = l0g,0=l0ge^ 



A polarisation method was employed in measuring the density. 

 Light from the source S was reflected from the plane mirror M, 

 and was viewed through 0, where it illuminated half of the field 

 of view. The other half of the field was illuminated by light from 

 S»^, which had been transmitted through a pair of Nicol prisms. 

 Metallic filament lamps in parallel were used as the sources of light. 



*5, 



o 



Fig. 1.— Polarization Photometer. 



