photographic rendering of Contrast. 637 



LXII. The Fundamental Law for the true photographic 

 rendering of Contrast. 



To the Editors of the Philosophical Magazine. 

 We must thank Mr. Ren wick for pointing out the three 

 "" obvious slips of the pen " in our paper. 



With regard to the terms " exposure," " printing light," 

 and " viewing light," the precise meaning is easily seen from 

 the context. Exposure always means intensity multiplied 

 by time, so also does printing light ; it is a matter of indif- 

 ference whether V and I are also similarly defined or taken 

 as intensities only. 



The u subject " is the distribution of light which falls on 

 the negative. To oo further back and consider the formation 



O O 



of this image by means of the camera would be to introduce 

 a complication which does not affect the operations with which 

 the plates are concerned. We purposely keep clear of this com- 

 plication ; it is easily introduced by those who desire to do this. 



We regret to have to say that we find it very hard to con- 

 sider seriouslv the remaining criticisms. The suggestion 

 that we have omitted to consider (on p. 100, line 4) " the 

 sensitiveness of the positive material used " is bewildering. 

 The determination of D 2 and logE 2 /s (amongst other things) 

 a determination of the sensitiveness of the positive plate. 

 No one knows this better than Mr. Renwick, who has devised 

 ingenious methods for determining this characteristic for 

 individual plates in connexion with plate sensitometry ! 

 What, then, does his criticism mean ? 



Nor can we treat seriously his remarks on the supposed 

 difficulty of representing various logarithms by means of 

 lengths on a diagram. It is very wrong dimensionally 

 of course; yet the same thing is done, and rightly done, on 

 ■every physical diagram in which any other quantity than a 

 length is plotted. 



We do not admit that the last complete paragraph on p. 191 

 can bear the interpretation which Mr. Renwick puts upon it : 

 namely, that K has something to do with the gradation of 

 the picture. At any rate, as he suspects, it is not intended 

 to have that meaning. Mr. Renwick considers that the law 

 we state to be fundamental for true gradation is ** more 

 stringent than is desirable." He replaces it bv the more 

 general law, I 6 .= KI " (or in our notation. I— KE,")' But 

 he is here confusing the requirements for true gradation 

 (viz. n = l) with arbitrary attempts which may be made to 

 make the best of a departure from true gradation when that 

 itself cannot be obtained with the photographic material at 

 •our disposal. Now, we know of no theoretical reason for 



