DISCOVERY 



231 



experimenters shows that the mcrease in size and sensi- 

 tiveness of the commercial crystals can be obtained 

 by two methods, one being the heating of the emulsion 

 to a considerable temperature, the other tlie digestion 

 of the emulsion at far lower temperatures in tht 

 presence of ammonia, which is a solvent, in a sHght 

 degree, of the silver'halide. But what actuall}- happens 

 to the crystals in the ripening process is still a mystery. 

 One can obtain, for instance, large crystals of compara- 

 tively low sensitiveness, and small crystals of very great 

 sensitiveness compared with the usual run of small 

 crystals. 



Slowing-up Fast Plates 



Examination of the sensitive crystals by Svedberg, 

 Toy, Sheppard and TriveUi, and others has shown 

 that on the surface of the crj^stals, and frequently 

 at the edges, are " sensitive spots " of possibly 

 adsorbed matter, and it is assumed that these 

 local centres are in some way responsible for the 

 increase in sensitiveness. Clark has recently found 

 that by treating " fast " crystals with chromic acid, 

 and thereby removing the sensitive spots, the^^ become 

 greatly reduced in speed, and that crystals of different 

 degrees of sensitiveness all appear to become reduced 

 to one common level of speed when so treated. This 

 would tend to indicate that ripened silver bromide, per 

 se, is of a very low sensitiveness (about 6 H. and D., or 

 probably much less), and that the speed of the plate 

 is obtained by depositing on the crystals as many as 

 possible of these reduction centres. \Miat they are is 



^ 



8 



'.^O" 





O 



o 

 O 





o 



o. 



o 



<\. 



^ 



• »^ o 



9 



« 





o ■ 



o 



^ 



Fig. I.— crystals of .\x kmulsion aftbr ripening to a spef,d 



OF 50 H. AND D. 



at present very uncertain. They must, of course, be 

 derived from impurities in the water, the gelatin, or 

 from the other raw materials used. The adsorbed matter 

 is extremely small, and may itself be supremely sensi- 



tive and easily ionised by light, or it may behave 

 purely as a catalyst, aiding the chemical change 

 without undergoing change itself. If the substance 



Fic. ;.— CRYSTAiS OF THE SAME EMULSION .\F'IER RIPENING 

 TO A SPEED OF 200 H. AND D. 



be an actual silver compound, its presence within the 

 crystal lattice may be discovered by X-ray analysis. 



Effect of Light on the Sensitive Silver 

 Bromide Crystals 



A point mucli in dispute at the moment is the 

 manner in which the sensitive crystals of silver bromide 

 are affected by light. Is the crystal more sensitive 

 the greater number of sensitive nuclei its surface 

 presents, or can the physico-chemical character of the 

 crystal itself make it more readily affected by the 

 nuclei, independently of their number (or size) ? 

 Controversy exists around the theories of the character 

 of light itself — whether it is a mere wave motion or 

 partakes of the character of innumerable darts as 

 propounded by Silberstein. If the light- dart theory 

 held good — and there seems at present sufficient ground 

 for doubting it — one would think that the area of the 

 crystal alone would decide its sensitiveness ; but this 

 is not the case, as has been already stated. 



Whatever be the actual procedure in exposure, and 

 whatever be the character of the latent image, we have 

 gained this knowledge as the result of recent research — 

 that the grain or crystal size and character control 

 the character of the emulsion, that one can to a great 

 extent predict the ultimate character of a plate by 

 previous micrographic analysis of the emulsion, and 

 that the sensitiveness of the silver bromide is due to 

 what for convenience we may call impurities deposited 

 on or adsorbed by the crystals.'*' That this knowledge 

 will lead to important progress in tiie art of emulsion- 

 making, and to the production of vastly more sensitive 



