675 



The fact to wliich I wish to draw attention is, that for every 

 pair of negatives the characteristic curve for light slopes considerably 

 more than the one for X-rays. Tiiis is clearly shown in fig. 1 — 4. 

 Each of them contains two curves, the upper one showing the 

 action of ligiit, the lower one the X-ray curve. The slope of these 

 cui-ves always proved to be diiferent in (he manner indicated and 

 to be independent of the development if onl}' both halves of the 

 plate were dexeloped in the same tray for (he same length of time 

 without undue restriction of the time. Changes in the development 

 merely caused changes in the slope of both curves at the same 

 time and in the same way. 



We may expect a physical difference in the action of light and 

 of X-rays on the photographic plate. The sensitive layer strongly 

 absorbs light, whereas X-rays are only slightly absorbed. As we 

 know that the action of both kinds of rays increases with the 

 intensity, we may in the case of light look for a strong action at 

 the surface of the sensitive layer and for a markedly diminished 

 action in the lower strata of the emulsion. In the case of X-rays 

 which are not notably weakened after passing through the silver- 

 bromide-emulsion we may reasonably expect that the action in the 

 deeper layers is not less than the action on the surface. After 

 development the reduced silver should be nearly ecpially deposited 

 in every part of the gelatine layer if the negative had been obtained 

 with X-rays. In light-negatives the silver would probably be 

 accumulated on the surface and only a slight amount would be 

 present in the deeper strata. Any one who has developed many lighl- 

 and X-ray-negatives knows, that with the former only the parts 

 exposed to the strongest lights are visible at the back after development 

 but before fixing, whereas properly developed unfixed K()Ntgkn- 

 negatives present nearly the same appearance at the back as on 

 thfe front surface. 



We may ask if this difference might be responsible for the 

 difference in the slope of the characteristic curves. It seems to me 

 that this is possible and even (juite probable, if we consider the 

 question in the following way. 



Let us first consider how the reduced silver is deposited in light- 

 and RöNTGEN-negatives. After this we shall see how this effects their 

 transparency. 



We may represent the absorption of light and RöNTGEN-rays in 

 the gelatinobromide-emulsion by the well-known formula 



J/^/.e-/-' (1) 



in wliich 7/ is the intensity of the radiation after passing a layer 



