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HANDBOOK OF PHOTOGRAPHY 



again moved until a balance of intensity is obtained in the eyepiece, when density of 

 the negative may be read directly from the calibrations of the steel tape. Calibration 

 of this tape involves the inverse-square law. 



Fig. 12. — Optical system of the Capstaff-Purdy densitometer manufactured by the 

 Eastman Kodak Company. The negative is placed on the stage, H, and light from the 

 lamp A is split into two beams. One beam passes directly through the negative to the 

 eyepiece, J, whereas the other travels from the lamp to the mirrors B, D, and F, where it is 

 reflected onto a split mirror G and then is reflected to the eyepiece J where a split field is 

 observed. When the wedge, W, is adjusted so that the two fields of the eyepiece match, 

 the density may be read off from the wedge calibration. The knurled screw, K, is used in 

 making adjustments for the reference position of the wedge. 



Capstaff-Purdy Densitometer. — The Capstaff-Purdy densitometer manufacturered 

 by the Eastman Kodak Company is an example of a subjective type of instrument 

 which depends for its indications upon a calibrated wedge. Figure 12 shows a diagram 





:.l- 



-/■ — 



-t^W~^\ 



^g 



--A-. > 





\ / / ^ / 



U-. 



Fig. 13. — Side and panel views of the Capstaff-Purdy densitometer. 



of the essential optical system of this densitometer, and Fig. 13 shows a schematic 

 diagram of the completed device. Here again the light from the lamp A is split into 

 two beams. One of these traverses from the lamp A through a diffusing glass U (over 



