PHOTOGRAPHIC SENSITOMETRY 



175 



which the negative is placed) and then passes through a field-forming photometer 

 head. This latter consists of the partly reflecting, partly absorbing, mirror F and the 

 mirror G, which has a circular disk from which the silver has been removed so that the 

 direct rays from the lamp may pass through it. The other beam traverses the path 

 from the lamp A to the mirror B through a graduated wedge C, a second mirror D, and 

 finally impinges on the partially reflecting mirror F, where it is reflected to the mirror 

 G and viewed together with the direct beam from A. Equality of the direct and 

 reflected beams is obtained by rotating the calibrated wedge W until a portion of the 

 wedge having suitable absorption intercepts the reflected light beam. The match of 

 the two fields is made by looking through the eyepiece / and rotating the circular 

 wedge. The initial adjustment for zero density (when the negative is removed from 



Fig. 14. 



Fig. 14. — External view of Eastman transmission densitometer. 



Fig. 15. — Optical parts as used in the Martens polarization photometer head. The 

 essential parts are: A, WoUaston polarizing prism; B, biprism for splitting the field; 

 C, analyzing Nicol prism; /, index pointer; Li, field lens cemented to Wollaston prism; L2, 

 eyepiece lens; 0, aperture for one field; 0', aperture for other field; S, scale. 



the densitometer) is made by moving the position of the lamp slightly by means of the 

 key i?. 



Figure 14 shows an external view of the completed instrument. The edge of the 

 wedge projects beyond one side of the case and may be rotated by the finger. The 

 wedge is made by copying on a photosensitive plate a circular wedge cast in gray-dyed 

 gelatin. Each such wedge must be calibrated against a standard before the densitom- 

 eter can be used to indicate density directly, and a suitable calibration scale must be 

 affixed to the circular wedge. The scale is read through a small circular window. 



Marten's Polarization Densitometer. — Another form of subjective densitometer is 

 the Marten's densitometer, which makes use of the variation in absorption of two polar- 

 izing prisms as their axes are rotated with respect to one another. A polarizing plate 

 or crystal, usually designated as a polarizer, is used for the production of a beam of 

 plane-polarized light. Another polarizing device, called the analyzer, is placed in 

 alignment with the polarizer. Through the relative rotation of the polarizer and 

 analyzer, the intensity of the light transmitted by the combination may be controlled. 



Figure 15 shows the schematic diagram of the optical parts of the Marten's polari- 

 zation photometer head, and Fig. 16 shows the complete instrument, including the 



