1904.] The Chemical Dynamics of Photographic Development. 449 



hydrazine, and NH 2 'OH, hydroxylamine, all of which function as 

 developers.* It seems possible that the quantitative investigation of 

 development might throw light on the affinities of a large group of 

 organic bodies. 



Owing to the complications present in organic developing solutions,! 

 the reaction was first studied with ferrous salts, those employed being 

 ferrous oxalate, citrate, and fluoride. These can be easily estimated 

 analytically, and disturbing side reactions do not occur to any extent. 

 The methods and conclusions arrived at can then be applied to the 

 study of other developing agents. 



Apparatus and Method of Enquiry. The apparatus used by the 

 authors and other investigators for the investigation of sensitometry 

 and development have been very fully described elsewhere,! so that 

 only a brief description with an account of the errors in working is 

 here necessary. 



The progress of the reaction was studied by measuring the density, 

 varying amounts of light action being obtained by means of a special 

 sensitometer. 



(a) The sensitometer impressed a known gradation of exposure oil 

 the plate by means of an accurately calibrated sector-wheel, exposed 

 to a constant pressure acetylene burner . 



(b) A special thermostat was employed, so that all the chemical 

 reactions involved took place under constant conditions, temperature 

 to 0-1 C. 



(c) The absorption photometer used was a Hiifner spectro- 

 photometer.|| The silver deposit had no spectral absorption, but 

 measurements were made in the bright green, the region of maximum 

 luminosity in the visual curve. II The instrument uses two Nicol 

 prisms for polarising the light, and so darkening the comparison field. 

 If 9 be the angle of rotation of the analyser, T, the transparency = I/Io 

 is equal to cos 2 #, and D, the density (after Hurter and Driffield) is equal 

 to -Iogiocos 2 0. The estimation of the probable error for a single 

 density the scatter gave the following result : The mean probable 

 error from 3'0 to O'lOO was approximately constant at O'OOT.** As 



* Le Roy., ' Bull. Soc. Fran?. Phot.,' 1894, vol. 23. Andresen, ' Phot. Corr.,' 

 vol. 36, p. 260. 



t Mees and Sheppard, ' Zeit. f. Wies. Phot.,' Bd. II, H. 1, 1904. 



J ' Phot. Journ.,' " Instruments for Sensitometric Investigation," vol. 44, 1904, 

 No. 7. 



Loc. cit. 



\\ Loc. cit. and ' Zeit. f. Physik. Chem.,' 3. 



IT Further, the region for maximum sensitiveness for small differences in 

 intensity, Unterschieds-empfindlichkeit, lies, according to Pfluger, between \ 495 

 and X 525. ' Ann. d. Phys.,' 4te Flge., Bd. 9, p. 207. 



** Cf. Martens : " Modified Konig. Spectrophotometer," ' Ann. d. Phys.,' 4te 

 Flge., Bd. 12, 1903. ' As long as a does not lie near or 90,^, the just 

 perceptible difference of brightness of the comparison fields, is constant." 



2 L 2 



