336 SECTIONAL TRANSACTIONS.— A. 



(product of illumination and time). For any material there is an optimum 

 rate of reception of energy, which varies with the temperature. While the 

 density increases with the exposure over a wide range, prolonged exposure 

 causes it to decrease (the region of solarization). The crystals in a layer 

 vary very much in sensitivity, which, in general, increases with their size. 

 The sensitivity is also governed by the nature of the surrounding medium 

 and by traces of foreign matter : of great importance is the adsorption 

 of certain dyes to the crystal faces thereby extending the natural sensitivity 

 of the silver halides from the Blue- Violet towards the Red and even into 

 the Infra-Red. 



Mr. S. O. Rawling. — Modern views of the action of developers in 

 photography (11.50). 



In making a photograph, development is necessary to make the latent 

 image visible. The unit is the crystal of silver halide ; the greater the 

 exposure the greater the number of developable crystals. The sensitivity 

 of a crystal is inversely proportional to the exposure necessary to bring it 

 across the threshold of developability. After application of a developer 

 to a crystal in an exposed plate a period of waiting ensues during which no 

 apparent change occurs ; then development begins at one or more points 

 on the crystal and spreads rapidly until the whole crystal is blackened. 

 Development centres are probably coincident with the units of the latent 

 image, which are believed to be particles of silver. The size of a particle of 

 silver probably determines whether or not it can initiate development. 

 The critical size has been variously estimated to lie between two or three 

 atoms of silver and some hundreds of atoms. Theories of developer 

 action are divided between two concepts : reduction of silver salts in 

 solution with subsequent deposition of silver on the latent image, and 

 adsorption of the developing agent by the latent image with subsequent 

 reduction of silver halide in the surface of the crystal. 



Prof. N. F. MoTT, F.R.S. — A theory of latent image formation (12.10). 



Ultra-violet light is known to colour rock salt crystals. The colour is due 

 to free electrons released by a kind of internal photoelectric effect, as may be 

 shown experimentally. The primary action of the light on the silver halide 

 grains in a photographic emulsion is similar ; electrons are liberated which 

 combine with the silver ions to form silver atoms. In the print-out process 

 these silver atoms coagulate to form metallic silver ; a detailed mechanism 

 of this coagulation is proposed. On the assumption that the latent image 

 is a submicroscopic speck of silver, it is possible on the basis of this mech- 

 anism to understand the reason for the breakdown of the reciprocity law 

 and also for the effect of temperature on the sensitivity of photographic 

 emulsions. 



General Discussion on The mode of action of the photographic plate {12. t,o). 



Afternoon. 



Mr. H. G. Green. — George Green, the mathematical physicist 

 (1793-1841) (2.30). 



The figure outstanding from the group of men, able and interested in 

 scientific matters and banded together through their association with the 



