( 11 ) 



of detail of the image. The action being slighter to the right, the 

 images are seen to be thinner accordingly. 



The same thing is to be observed in a Runtgenogram placed at 

 my disposal by P. H. Eykman. The plate was exposed to Röntgen 

 rays with a so-called intensification screen (calcium wolframate screen), 

 and laid aside some time before development. Calcinm wolframate 

 postluminesces l ) owing to which a plate is consequently further 

 exposed. The objects photographed with Röntgen rays were a piece 

 of bone, a piece of thin and a piece of thicker, insulated copper 

 wire. Fig. 7 gives a reproduction of the negative. The places of the 

 thin copper wire show development of the silver bromide which is 

 nearly as strong as in the field, where the Röntgen rays and the 

 luminescence together have acted most strongly. A narrow strip 

 along the edge indicates how far the screen covered the plate ; con- 

 sequently there only the R. rays have acted. The place of the thin 

 copper wire does not show a trace of development there ; conse- 

 quently the exposure remained below the liminal value. 



The development of this place cannot be put down to irradiation 

 through the screen; in that case the same thing would have to be observed 

 in the case of the thick copper wire and the edge of the screen. 

 Consequently the R rays must really have acted in such a way in 

 that place, that the screen luminesced and this acted upon the 

 silver bromide, while the exposure to R. rays remained below the 

 liminal value. 



P. H. Eykman also showed me a negative in the case of which the 

 screen after irradiation had only been brought into contact with an 

 unexposed plate 2 ). It showed a very thin image, from which it follows 

 that the strongest action of the screen takes place immediately after 

 the transformation of the absorbed Röntgen energy. Consequently if 

 in the case of a runtgenogram with a calcium wolframate screen 

 the action of the R. rays could be prevented, much shorter exposures 

 would be sul'ticient to produce a good image. Hitherto this has 

 proved to be impossible. 



Another fact important for our knowledge of the latent image 

 may be gathered from fig 6. The right bottom corner shows that 

 the thinner the fog of diffused exposure becomes, the thinner also 

 the polarized copy is. From this it follows that in the case of secon- 

 dary exposure the liminal value is lower than in the case of pri- 

 mary exposure, i.e. the amount of light necessary to effect the begin- 



i) Fortschr. a. d. Geb. d. R-Str. 1901; Bd. IV; S. 180. 



2 ) The calcium wolframate screen was exposed to the R rays at the same 

 time as the negative of Fig. 7. 



