MOUNT WILSON SOLAR OBSERVATORY. 205 



photographic plates, for with the faint hghts involved the exposure time is 

 by far the greater part of that required for all the various manipulations. 

 Doubling the speed of the plate, therefore, practically doubles the amount 

 of work that may be done with a costly instrument. Economic considera- 

 tions alone would accordingly seem to require that every effort be made to 

 secure improvements, while the increase in efficiency in the broader sense 

 of being able to extend investigations into fields not now accessible is a 

 reason that speaks for itself. 



In order to obtain some idea of the present status of photographic manu- 

 facture and of the possibility of a further perfection of plates and processes, 

 Mr. Scares has recently interviewed various photo-chemists and manufac- 

 turers both in the United States and in Europe. The results of this inves- 

 tigation may be summarized as follows : 



The photographic industry has been developed by empirical methods to 

 an even greater degree than most other industries, for the processes in- 

 volved are among the most obscure in the whole range of physical chemis- 

 try. Even to-day there is no agreement among those best informed as to 

 what actually takes place when the emulsion is ripened, or when the ripened 

 and highly sensitive emulsion is exposed to the action of light. Nor is it 

 understood why the emulsification of silver bromide in gelatine leads to a 

 far more sensitive plate than can be obtained through the use of collodion. 

 The chemistry of development is better understood, but even here there are 

 many obscure points. 



In the absence of more definite knowledge of these fundamental phe- 

 nomena it seems clear that plate manufacture must remain the art that it is 

 to-day, and that improvements will be more or less a matter of chance. On 

 the other hand, even a partial solution of the problem of ripening proba- 

 bly would afford a basis for a rationally directed inquiry after improved 

 methods. 



As for the more specific possibilities in the way of improvement, the fol- 

 lowing may be stated : The sensitiveness of the emulsified silver salt de- 

 pends upon the absorption of the light which falls upon it. Without such 

 absorption there can be no beginning of the action which with the applica- 

 tion of a developing agent ends in the reduction of the salt to metallic silver. 

 Apparently the relatively unstable molecules of the silver salt may be re- 

 garded as resonators. Ripening renders the molecules more susceptible to 

 the disruptive action of the vibrations to which they are attuned. In prac- 

 tice this seems invariably to be accompanied by an increase in the size of 

 the particles or grains suspended in the emulsion. Whether or not this 

 result is necessary is at present uncertain, though it is certain that a coarse- 

 grained emulsion does not necessarily possess high sensitiveness. 



It is significant, however, that the application of various dyes increases, 

 without any increase in the grain, the sensitiveness of the ripened emulsion 

 for wave-lengths for which normally it possesses no special sensitiveness. 



