154 True Photographic Rendering of Contrasts. 



exposure is given, the characteristic curve starts at K with 

 zero density. All other points are easily deduced, as shown 

 for 1/ and M', thus : — Since the lengths V and m' represent 

 density differences (and therefore logE differences) in the 

 negative by means of which the densities L' and M' respec- 

 tively are to he secured in the print, it is clear that the 

 characteristic curve of the positive material must have 

 density values equal to L' and M' at the logE values distant 

 from K by the lengths V and m! respectively. 



It is, of course, equally easy to deduce a negative cha- 

 racteristic curve, given the positive characteristic curve and 

 the final print. 



It will be obvious that Porter and Slade's " conjugate "" 

 curves are simply a special case in which the gradation of 

 the desired print is represented by a straight line rising 

 at 45° (gamma = 1) from H. 



Addendum. — I regret that in my remarks on Porter and 

 Slade's paper, I neglected to state that the validity of the 

 authors' neat proof of the necessary relations between 

 positive and negative, if exact reproduction is required, is- 

 unaffected by the obscurities and omissions to which I 

 desired to call attention. I have no desire to carry on 

 a controversy concerning what are, after all, relatively minor 

 points, and will therefore simply state my opinion that the 

 authors themselves evidently regarded both I and E as 

 intensities when writing their fundamental equation, I = KE r 

 as is shown by the sentence near the bottom of p. 191 : — 

 " The more nearly K is to unity the more nearly will the 

 light from the positive be not only the same in gradation as 

 that from the subject but also the same in absolute intensity" 



For those concerned with the practical application of such 

 studies, their value would, I am sure, be much enhanced by 

 a clearer statement of the meanings of the terms employed 

 and by the exercise of a little more care when giving an 

 outline of the current state of knowledge on the subject. 



The power law attributed to me is not mine. I do not 

 know its origin but, like many others, have simply accepted 

 it as being, usually, the least objectionable departure from 

 exact reproduction when the latter is impracticable. — 

 F. F. H. 



