( 28) 



camera). On one hand this seems an objection, as all printing methods 

 are based upon the production of normal copies (the negative process). 

 But it should be borne in mind that hitherto very few researches 

 have been made in this domain. 



For direct colour photography *) with colour elements lying side 

 by side under the emulsion, according to L. Ducos du Hauron's system 

 (which especially lately has given promise of a great future), which 

 requires directly polarized copies, and which so far has only succeeded 

 in obtaining them in an indirect way, this method would also be 

 practically valuable. 



In this direction little experimenting has hitherto been done from 

 a photo-chemical point of view, and even in what has been done 

 it has been impossible to account for the phenomena that occurred, 

 so that for the present there is no need for us to take too pessimistic 

 a standpoint with reference to this. 



VII. The shape of the image in the case of HerscheVs effect. 



As to the shape of the image in the case of Herschel's effect 

 fig. 8 may be referred to. 



It is clear that after the critical exposure the normal copy is 

 again a surface image. If a considerable portion of the surface 

 silver haloid present has already been decomposed into « silver 

 subhaloid and halogen, the secondary exposure will not be able 

 again to form as much « silver subhaloid as would have been 

 the case if the primary exposure had not taken place. A negative 

 is obtained then the density of which is less than that of a plate 

 not previously exposed. This case presents itself in the sky a in fig. 5. 



Advanced primary exposure may result in solarization, in which 

 case the surface silver haloid can no longer supply the same quantity 

 of germs as was present before: Herschel's effect then coalesces 

 with solarisation, and the critical exposure can no longer be ascertained. 



The greater light sensitiveness of the « silver subhaloid as compared 



!) I expressly call this method "direct", because I cannot agree to the judgment 

 of a number of others, who want to classify it among the indirect methods. They 

 say that it is not direct colour photography, but three-colour photography, 

 ignoring the fact that the bleaching method which is reckoned to belong to the 

 direct methods, is also three-colour photography. Nor can I agree to A.v.Hübl's 

 classification (Phot. Rundschau, 1908, p. 2), by which the bleaching process 

 would be assigned to the indirect methods. The fact of the matter is that the 

 difference is only a question of method, i.e. whether the colours are obtained 

 directly after exposure (with development), or only through subsequent addition. 



