Rendering r of Photographic Contrast. 633 



on mixture. Electric and magnetic fields have no influence 

 on surface combustion that can be observed by the phe- 

 nomena of ignition. 



Compared with the varied phenomena of spark ignition 

 iiot wire ignition is singularly constant in type. A further 

 examination of the electrical and thermal phenomena of 

 surface combustion is to be desired. 



LXI. The Fundamental Law for the true 'photographic render- 

 ing of Contrast. By F. F.' Ren wick, A.C.G.I., F.I.C.* 



SIXOE the publication in May 1890 of their first paper in 

 the Journal of the Society of Chemical Industry, the 

 views of Hurter and Driffield concerning the scientific 

 principles which should guide photographers in the making 

 of negatives and positives, have been the target for much 

 criticism and the focus of many controversies. 



In a paper by Prof. A. W. Porter, F.R.S., and Dr. P. E. 

 Slade in the July issue of the Philosophical Magazine 

 (vol. xxxviii. p. 187, July 1919) there appears a review 

 of the fundamental principles of picture- making which 

 claims to present the whole subject in a new and truer 

 •aspect and attributes to Hurter and Driffield "as the 

 result of a somewhat quick judgment''' .... "an imperfect 

 •conception of the true conditions for securing a true repre- 

 sentation of gradation." 



It appears to the present writer unfortunate that at the 

 time they wrote their paper Porter and Slade were un- 

 acquainted with both Lord Rayleigh's paper (Phil. Mag. 

 xxii. p. 734 (1911), reprinted Brit. Journ. Phot, lviii. 

 p. 991 (1911)) and with the 1916 Traill Taylor Memorial 

 lectu e before the Royal Photographic Society (Phot. 

 Journ. vol. Ivi. p. 222 (916)) to which they refer in their 

 last two paragraphs, or they would doubtless have dealt 

 less fully with some points which have long since received 

 general acceptance and more fully with others on which 

 they seem to hold different views from most of those 

 who have worked at photographic science during the last 

 decade or two. 



I propose later to describe my method of deducing the 

 ""reciprocal''' curves employed in the 1916 Traill Taylor 

 lecture, and to show that it lias a more general applicability 



* Communicated bv the Author. 



