638 Rendering of Photographic Contrast. 



supposing that the power-law will give a better approximate- 

 representation than any other law, e.g. a sine-law. All that 

 can possibly be claimed for it is precisely what can be claimed 

 for any small deviation from the law I = KEj ; namely, that 

 such deviation will not be of serious importance. Mr. Ben wick 

 appears to think that by the choice of n he is able to "compress 

 the gradation in a print of a subject whose range of contrasts 

 was approximately greater than the printing medium will 

 yield." We know of no way in which this can be done. Most 

 certainly it cannot be done by arbitrarily altering a formula. 

 So that when he asks whether we " concur with the generally 

 adopted rule, ' we say that we most decidedly do noiconcur.. 

 We are assuming in making this statement that the Weber- 

 Fechner law connecting visual sensation with the physical 

 stimulus is correct. It is so throughout an exceedingly large 

 range. For very small intensities it appears to break down : 

 and in this region some modification of the law is required. 

 It may be possible to represent this by taking n variable in a 

 definite icay instead of unity. This is, however, a different 

 problem from that indicated by Mr. Ren wick. Further, we 

 do not take into account the fact that in many cases true 

 rendering is not required. A dull landscape can be brightened 

 up wonderfully by departing from the law for strict repro- 

 duction. Such aesthetic questions are quite foreign to the 

 subject of our paper. 



We shall be very pleased if it turns out that we have mis- 

 taken Messrs. Jones, Nutting, and Mees in the passage we cited. 

 It is somewhat obscure. In any case there is no question 

 of the accuracy of the considerations on p. 195 and fig. 8 on 

 the matching of ranges in the negative and positive plates. 



Finally, we have in our paper duly acknowledged both the 

 late Lord Rayleigh's and Mr. Renwick's papers dealing with 

 the same subject. What does it matter that we did not 

 know of these papers until our paper was being copied out? 

 If we had found our conclusions were identical with those of 

 our predecessors, there would be no need to go further. But 

 we found after full consideration of both papers that we 

 were at variance with them. We were at variance with 

 Lord Rayleigh because he made assumptions in his parti- 

 cular cases which did not accord with the properties of 

 photographic plates. We wpre at variance w ith Mr. Renwick 

 because of his power-law which we consider to be funda- 

 mentally wrong ; and moreover we were uncertain of his 

 reciprocal curve diagram because it is accompanied by 

 no explanation. Even now he has not vouchsafed any 

 explanation. Alfred W. Porter. 



September 18th, 1919. R. E. SLADE. 



