448 Messrs. S. E. Sheppard and C. E. K. Mees. [Dec. 20, 



according to Hurter and Driffield is directly proportional to the mass 

 of silver per unit area.* 



References to the work on exposure and development will be made 

 in the course of the present work. Many of their conclusions have 

 been controverted! chiefly from isolated experiments. Nevertheless, 

 their method of sensitometry has been nominally adopted for com- 

 mercial purposes in England. It has, however, been attacked by 

 Dr. Eder| in the course of his important researches on sensitometry. 



The authors have dealt elsewhere with the question of sensitometry, 

 with the result of confirming in general the results of Hurter and 

 Driffield. 



Applications of physico-chemical doctrine to photographic phenomena 

 may be found in the short-lived ' Archiv f. wiss. Phot.' and, in 

 especial, Abegg's " silver-germ " theory of exposure and development. 

 This is dealt with later. 



All developers are, chemically speaking, reducing agents, but the 

 converse does not hold. In connection with this, Bredig|| has pointed 

 out the importance of the " reduction potential "U as a function of 

 the developer. However, as there is no strict proportionality between 

 " potential " and reaction velocity,** other conceptions are necessary, 

 if the " efficiency " of developers is to be measured. The relation, 

 as has been pointed out, may be conceived as analogous to Ohm's law, 



velocity = . . ,tt and this " resistance " in chemical reactions is 

 resistance 



a term difficult to define or measure. 



The chemistry of organic developing agents has been extensively 

 investigated by Messrs. Lumiere and Dr. Andresen,J| to whom we 

 owe the following rule : All organic developers are substituted 

 aromatic derivatives, containing two of the groups OH and NH 2 , 

 joined by an ortho- or para- linking, the meta- bodies not acting as 

 developers. Andresen has pointed out the analogy between these 

 bodies and the simple inorganic types, 02H2, hydrogen peroxide, N2 



* Cf. Eder : * Beitrage zur Photochemie u. Spectral-analyse,' 1904 ; Luther, 

 ' Zeit. f. Phys. Chem.,' 1900 (-Re/.) recommends the term " Extinction," instead 

 of the anomalous " density," as this quantity is the same as the " Extinction 

 coefficient " introduced by Vierordt in absorption-photometry. 



t For a bibliography of this controversy, see C. E. K. Mees and S. E. Sheppard, 

 ' Instruments for Sensitometry," ' Phot. Journ.,' vol. 44, No. 7, p. 222 ; and 

 " On Sensitometry," ibid.. No. 9. 



J Loc. cit. supra, or ' Sitzber. d. Wien. Akad.,' vol. 113, 1899, sect. A. 



' Archiv f . Wies. Phot.,' 1899 to 1900. 



||. Eder's Jahrbuch, 1895. 



1 Bancroft and Neumann, ' Zeit. f. Physik. Chem.,' 10, 3S7. Nernst, < Theoret. 

 Chem.,' 4te Autiage, p. 710. 



** Ostwald, ' Lehrbuch,' 2te Bd., 2te Tl. (2te Auflage). 



ft Nernst, c Theoret. Chemie.,' 4te Auflae, p. 656. 



jj ' Photo. Bulletin,' 1895, et seq. Eder's Jahrbuch, 1899 (pp. 140 to 147). 



' Phot. Corr.,' 1899, p. 212. Eder's Jahrbuch, 1899 (pp. 140 to 147). 



