Chemistry and Physics. 81 



would give rise to a similar band, if the metal (in the form of foil) 

 were placed close to the sensitive film instead of being located in 

 the gelatine film itself. Layers of pure tinfoil were pressed flat 

 against the back of the photographic plate and exposures were 

 made with the sensitive film turned away from the incident radia- 

 tion. Under these circumstances the soft, secondary radiation 

 starting in the tinfoil would be absorbed by the glass and thus 

 prevented from confusing the negative. As anticipated, a tin 

 band of slightly shorter wave-length than the supposedly silver 

 band is a prominent feature of the photograph. The new band 

 ends abruptly on its longer wave-length side and is similar in all 

 respects to the three bands of the first negative. In order to 

 decide whether the tin band was due to secondary Rontgen rays 

 or to swift electrons, the last experiment was repeated under 

 slightly different conditions. A sheet of aluminium foil, of such 

 a thickness (0 - l mm ) as to absorb all corpuscular radiation but to 

 readily transmit X-rays, was interposed between the tinfoil and 

 the sensitive film. For sake of comparison, a longitudinal slot 

 was cut in the aluminium screen. The tin band was of the same 

 intensity behind the aluminium as on the unscreened strip of the 

 negative. Therefore the tin band was due to the strong second- 

 ary or fluorescent Rontgen radiation, excited in the tin by the dis- 

 persed primary rays. The fact that the secondary rays give a 

 continuous band instead of one or more homogeneous lines shows 

 that the band corresponds to the entire spectral interval within 

 which the fluorescent radiation can be excited. It is worthy of 

 note that the intensity of the secondary radiation near the edge 

 of the tin band was of the same order of magnitude as the inten- 

 sity of the primary beam. Accordingly the primary rays must 

 experience very strong absorption in the tin. When an experi- 

 ment was performed with sheets of tin placed a few centimeters 

 inf ront of the photographic plate, it was found that the absorp- 

 tion band had exactly the same edge and complementary inten- 

 sity distribution as the fluorescent emission band. Having obtained 

 this information about tin, it was easy for the investigator to show 

 that the supposedly silver bands were really due to the silver in 

 the photographic emulsion. A narrow strip of pure silver foil 

 (0 - 013 mm thick) was placed at a distance of l cm before the plate and 

 an exposure taken. The characteristic edge of the silver band dis- 

 appeared almost completely and, for each wave-length, the selec- 

 tive absorption in the silver foil had produced precisely the 

 complementary photographic action as the selective intensification 

 of the silver inside the gelatine. Although Wagner has not yet 

 demonstrated experimentally that the " bromine band " owes its 

 origin to the presence of bromine in the sensitive film, neverthe- 

 less the preceding work leaves but little doubt as to the correct- 

 ness of the hypothesis. It may be concluded, therefore, that the 

 photographic action of Rontgen rays of very short wave-length 

 is due to the highly selective fluorescence of the silver and bro- 

 mine atoms. On the other hand, the ability of the plate to record 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 235. — July, 1915. 

 6 



