The Chemical Dynamics of Photographic Development. 447 



" The Theory of Photographic Processes : On the Chemical 

 Dynamics of Development." By S. E. SHEPPARD, B.Sc., and 

 C. E. K. MEES, B.Sc. Communicated by SIR WILLIAM 

 BAMSAY, K.C.B., F.E.S. Eeceived December 20, 1904, Eead 

 February 2, 1905. 



(Presented in Theses for the Degree of B.Sc. by Research in the University of 



London.) 



Historical and Introductory. The following work was undertaken with 

 the view of applying physico-chemical methods to the study of photo- 

 graphic development. Although, as has been pointed out by Ostwald, 

 the problem falls in the province of chemical dynamics, but little 

 systematic work has been done from this point of view. The increasing 

 use of gelatino-bromide films in connection with radiation phenomena 

 make a greater knowledge of the laws describing development and 

 exposure very desirable, while the importance of the photographic 

 industry needs no comment. The following brief historical introduc- 

 tion is necessary in order to sum up the progress made and the 

 terminology in use : 



In 1878, Sir W. Abney* showed that the black reduction product 

 in development was metallic silver. He introduced the measurement 

 of the transparency of the deposits by means of a photometer, and 

 proposed as the relation between the mass of silver reduced and the 

 transparency a form of the law of error. In 1890, Messrs. F. Hurter 

 and V. C. Drimeldf made a systematic survey of exposure and develop- 

 ment in which a new terminology was introduced and several very 

 important conceptions and results. These were extended in 1898 in 

 a second paper dealing largely with development.! They considered 

 that the ordinary exponential law for the absorption of light in 

 homogeneous media held for the photographic image and confirmed 

 this view experimentally. They gave the following terminology, which 

 has been generally accepted : 



I Intensity transmitted 



Transparency ... 1 = =- = ^- ~ .-, > 

 I Intensity incident 



Opacity = ^ = ^ , 



Density D= - log e T = log, 0. 



D r the density, is for convenience usually taken as - logio T, 



* ' Phil. Mag.,' 1878 ; also ' E. B.,' 10th edit., art. " Photography." 



f ' Journ. of Soc. of Chem. Industry,' May, 1890. 



J ' Photographic Journal,' 1898 ; Eder's ' Jahrbuch f . Phot.,' 1899. 



Bumen and Eoscoe ' Pogg. Ann.,' vols. 95 to 100. 



VOL. LXXIV. 2 L 



