L 187 1 



XIV. The Fundamental Law for the True Photographic 

 Rendering of Contrast. By Alfred W. Porter, D.Sc, 

 F.R.S., University College, and R. E. Slade, D.Sc.,, 

 British Photographic Research Association *. 



THE modern treatment of the character of a photographic 

 plate is based entirely (in England at any rate) upon 

 the method of examination introduced by F. Hurter and 

 V. C. Driffield in a paper read before the Society of Chemical 

 Industry and printed in their Journal dated May 31st, 1890.. 

 The advance that was made by these authors was so great 

 and the utility of their method so advantageous compared 

 with previous methods, that the whole photographic world 

 has joined in according them the honour clue to their insight 

 into the problem. 



Nevertheless, a certain malaise is often felt in regard to- 

 the logical foundation of their method. Perhaps they 

 themselves are to blame for this, because one of the important 

 steps in their chain of arguments is made in so cursory a 

 fashion (in a single sentence) that any reader must simply 

 assume it to be correct unless he himself develops the 

 argument in all its detail from the beginning. 



The whole argument is this")*: — " Since the density is the 

 logarithm of the opacity, and since in a theoretically perfect 

 negative, the opacities are directly proportional to the 

 intensities of the light which produced them, it follows that 

 each density must be proportional to the logarithm of the- 

 light intensity which produced it." They add in brackets :. 

 " More correctly the density is a linear function of the 

 intensity of light and time of exposure/'' 



Unfortunately in making this detailed examination one of 

 us (A. W. P.) has recently discovered that their principal 

 conclusion is erroneous when regarded as a general principle. 

 We shall first of all describe such a detailed examination 

 and then discuss in what respects difference is found from 

 Hurter and Driffield's results. 



The problem deals with the taking of a negative followed 

 by the printing of the negative on to another plate or paper, 

 and investigates the conditions under which the final positive- 

 gives a true rendering of the contrasts in the original subject,. 

 There are several parts to it. 



iors. 



* Communicated by the Authc... 



t Journ. Soc. Chem. Ind. May 30th, 1890. 



