( 27 ) 



trally highest. From M. Andresen's experiments : ) it appears that the 

 photo-chemical decomposition products remain in contact \vith~the 

 dyestuff, so that the a silver subhaloid obtains a different colour sen- 

 sitiveness. In this case complications may occur, if the dye is at the 

 same time an absorbent of halogen (a chemical sensitizer)," by which 

 it changes or loses its absorption spectrum, and a consequent promo- 

 tion of the photo-chemical decomposition action sets in. 



VI. Conclusions. 



From what has been said a few conclusions may be drawn, 

 which may be of importance in practice. 



Both a silver subbromide and iodide are substances of a much 

 greater sensitiveness to light than the corresponding silver haloid. 



Consequently if it was possible to compose emulsions in which 

 these substances were present side by side with the silver haloid 

 which as feeding substance is indispensable for development, plates 

 would be obtained not only of a higher sensitiveness than the present 

 ones, but in them a chemical sensitizer would be practically desirable 

 in every respect to prevent regression. Such plates would entirely 

 comply with the reciprocity rule, and would render the light grada- 

 tions of the objects to be photographed much more correctly, which 

 m&y be of great value to astronomical photography, e.g. for the 

 determination of the light intensity of stars by the photo-chemical 

 method (Photometry). 



The a silver subhaloid can be optically sensitized, so that its ap- 

 plication might obtain a great extension. The exact colour sensiti- 

 veness of the a silver subhaloid separately is not yet known exactly. 

 (That in the case of secondary exposure the highest sensitiveness is 

 situated in the red, the lowest in the green, points with great pro- 

 bability to the a silver subhaloid being a green substance). The 

 experiments mentioned indicate everywhere only the difference in 

 light sensitiveness between silver haloid and the a silver subhaloid. 

 The greater this difference, the more favourable the result obtained. 

 Consequently the best expectations might be entertained with respect 

 to silver chloride plates with « silver subiodide, and it is an open 

 question whether the latter may not be allowed to ripen too. The 

 /^-silver subhaloid seems to possess, photo-chemically, an extremely 

 low sensitiveness, which can only be advantageous in practice. 



This process yields directly polarized copies (positives through the 



G ) Phot. Korresp. 1898; S. 504. 



