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HANDBOOK OF PHOTOGRAPHY 



indicates a speed twice that of the 23° Scheiner, while 1300 H and D is twice as fast as 

 650 H and D. 



The Watkins speed scale, like the H and D, is based on the inertia of the photo- 

 graphic material but uses for k the number 68 

 instead of 34 chosen by Hurter and Driffield 

 or the value 10 frequently used in the United 

 States. However, L. A. Jones (" Photographic 

 Sensitometry") reports that the Watkins con- 

 stant, as determined from measurements on 

 emulsions, is more nearly 50 than 68. 



The Wynne system is another one of 

 several in which plate speed is specified in 

 terms of inertia and differs from the others in 

 the units selected for the specification of film 

 or plate speed. The numbers of the Wynne 

 system are expressed in terms of lens aperture, 

 as indicated by the symbol F which precedes 

 the Wynne speed number. The Wynne 

 ratings are proportional to the square root of 

 the Watkins number multiplied by 6.4. Thus 

 a Watkins speed of 64 corresponds to a Wynne 

 speed of 51.2. 



For many purposes and under many conditions, the expression of speed in terms of 

 inertia is of great value. As long as all the straight-line portions of a family of 

 D-logio E curves pass through a common intersection point which lies on the logio E 

 axis, inertia and hence speed are independent of development time. Under such 



Log.oE 

 Fig. 25. — D-logio E curves showing 

 that two emulsions might have the same 

 threshold speed (as determined by the 

 point at which their curves reach the 

 abscissa) but quite different inertia 

 values. The differences are due to the 

 curvature of the toe of the curves. 



Fig. 26. — D-logio E curves with depressed common intersection point, illustrating 

 that the inertia speed of a film may depend upon the degree of development which it 

 receives. 



conditions the speed becomes a very significant constant for the photographic material. 

 Unfortunately, the existence of a common intersection point lying upon the logio E 

 axis is frequently not found in practice. In most cases of normal development a 

 common intersection point is found, provided that proper corrections have been made 

 for fog. This intersection point very frequently lies below the logio E axis and in 



