November 30, 1917] 



SCIENCE 



523 



of the sensitive material itself, which in the 

 case of modern photographic plates, films 

 and paper is called the emulsion, is a prov- 

 ince of colloid and physical chemistry, col- 

 loid chemistry dealing with the precipita- 

 tion and nature of the sensitive silver salts 

 formed in their gelatine layer, while phys- 

 ical chemistry informs us as to the nature 



fore deals with sensitometry and the theory 

 of exposure, the chemist must deal at the 

 same time with the theory of development 

 and with the conditions relating to the de- 

 velopment of photographic images. 



A laboratory, therefore, for the study of 

 photographic problems must be arranged 

 with a number of sections such as are 



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of the reactions which go on, both in the 

 formation of the sensitive substance and in 

 its subsequent development after exposure. 

 The organic chemist prepares the reduc- 

 ing agents required for development and 

 the dyes by which color sensitiveness is 

 given to the photographic materials and by 

 which the art of color photography can be 

 carried on, and while the phj-sicist there- 



shown in Fig. 2. In physics we require 

 departments dealing with sensitometry and 

 with illumination, reflection and absorption, 

 colorimetry, spectroscopy and geometrical 

 optics. "We need a department of colloid 

 chemistrj', one of physical chemistry, one 

 of organic chemistry, one of photo-chemis- 

 try to deal with the action of light upon 

 the plate, and finally a num.ber of photo- 



