for the true Photographic Rendering of Contrasts. 153 



characteristic curve (below B) of the positive material is 

 employed to reproduce the tone corresponding to the point P 

 of the original. 



All other points on the required curve (dotted in the 

 diagram, fig. 1) are now easily fixed in the following way: — 

 Consider any point Q on the original wedge ; the length I 

 lying above this point between AA and the negative cha- 

 racteristic curve, represents the density difference, and 

 therefore the log E difference, available for printing. Measure 

 off I along the exposure axis from the starting point B ; 

 read off the density value on the positive characteristic 

 curve above the point so found and transfer it as an ordinate 

 to the point Q ; and so on for as many points as desired. 



Case II. — In fig. 2 let the continuous curves represent 

 the characteristic curve of the negative material and the 

 desired gradation of the finished print respectively. It is 

 required to find what form of characteristic curve for the 

 positive material would be necessary to obtain the desired 

 result. 



The total density range of the negative is represented by 

 the height R on the ordinate scale ; hence the total logE 

 range of that part of the positive characteristic curve which 

 concerns us is represented by an equal length on the exposure 

 axis. For the sake of: clearness this has been set off from 

 the right side of the diagram as HK. 



No density being required in the print where the least 



