206 REPORTS ON INVESTIGATIONS AND PROJECTS. 



The addition of the dye increases the resonance effect for the vibrations in 

 question, but the grain remains unchanged. There seems no a priori reason, 

 therefore, why it should not be possible to start with a relatively fine-grained 

 emulsion, and through the addition of a properly chosen substance increase 

 the absorption for those rays for which it is primarily sensitive — the blue 

 and the violet — to such an extent as to make it comparable in sensitiveness 

 with the ordinary high-speed, coarse-grained plate. There seems to lie in 

 this direction an avenue for profitable research, and apparently it possesses 

 the advantage that such investigations need not wait for the solution of the 

 extremely difficult problem of ripening. 



The investigation of developing agents also ofifers a field for investiga- 

 tion, for it is by no means clear that the last word has been said in this con- 

 nection. It is perhaps too much to hope for a discovery comparable with 

 that of the method of alkaline development, but undoubtedly improvements 

 bearing directly upon the sensitiveness and the grain of the plate are still 

 to be made. 



It is a matter of common experience that plates of high speed are lack- 

 ing in contrast. They solarize without attaining high density. This, how- 

 ever, seems to be a result of the coarseness of grain rather than of the 

 speed. Simple geometrical considerations show that the covering power or 

 density attainable with a given emulsion must depend directly on the fineness 

 and distribution of the particles of reduced silver, and apparently the solu- 

 tion of the problem of preparing rapid plates of fine grain will bring with 

 it the high contrast desirable for many lines of astrophysical investigation. 



As already suggested, sensitiveness for special regions may be derived by 

 staining the emulsified silver salt with dyes which cause the molecules to ab- 

 sorb those rays for which sensitiveness is required. We are already in the 

 possession of a combination of dyes well adapted to the production of a gen- 

 eral spectral sensitiveness for the entire visible region in the plate most com- 

 monly used. This requires modification to adapt it to other types of plate 

 used for various investigations; and it is highly important that a special 

 search should be undertaken for dyes which will bring the infra-red and 

 the extreme ultra-violet more easily within the range of photographic repro- 

 duction. Efforts should also be made to reproduce, and improve if possi- 

 ble, the methods used by Abney thirty years ago for the preparation of the 

 red-sensitive emulsions with which he was able to photograph farther into 

 the infra-red than has since been possible. 



The above are but a few of the various questions that require study. 

 Numerous others might easily be added, such, for example, as the properties 

 of different kinds of gelatine and of emulsifying media other than gelatine; 

 the conditions which determine the accidental and at present apparently 

 uncontrollable differences in sensitiveness shown by emulsions prepared by 

 the same formula ; and the preparation of emulsions with various combina- 

 tions of the silver haloids, particularly the bromide and iodide of silver. 



