( 22 ) 



and consequently that the «-silver subhaloid is a substance sensitive 

 to red. If « silver bromide gelatine plate is exposed to the action of 

 a continuous spectrum, the reducibleness in the case of increasing 

 exposure will proceed from blue to red. While after development it 

 is observed that with increased exposure the density of the plate 

 increases about and in the spectral blue, the yellow, the orange, 

 and especially the red obtain only very slight densities. Consequently 

 it is evident that the cause why the density of the image in the 

 red, yellow, and orange portions cannot increase above a maximum, 

 which is very low, lies in the a silver subhaloid possessing a far 

 greater sensitiveness to red than the silver haloid, so that very soon 

 a state of equilibrium has been reached, in which in a progressive 

 process as much «-silver subhaloid is formed as destroyed. 



The substance of the developable latent image is considered identical 

 with M. Carey Lea's photohaloid. Now how does this behave in 

 red light? 



M. Carey Lea : ) exposed his pink photohaloid to the action of a 

 spectrum ; while under all colours the photohaloid changed, it remained 

 unchanged in the red. From this it appears that the subhaloid germ 

 of the latent image must be another substance than M. Carey Lea's 

 photohaloid. 



The behaviour of Röntgen rays differs from that of other kinds 

 of light. According to P. Villard, R. W. Wood, R. Luther, and 

 W. A. Uschkoff they show no Herschel's effect in the case of 

 secondary exposure. This cannot be ascribed to total non-sensitiveness 

 of the a silver subhaloid to Röntgen rays. It is true, F. Hausmann 2 ) 

 and others stated that Röntgen rays produced no solarization, and 

 consequently that there was no formation of ,? silver subhaloid, but 

 P. H. Eykman s ), and subsequently K. Schaum and W. Braun 4 j 

 could show that they do. So the silver subhaloid is also sensitive to 

 Röntgen rays, and the non-appearance of Herschel's effect must be 

 put down to the cause that for Röntgen rays the silver haloid has 

 a liminal value as great as, or greater than the photochemical 

 induction of the a silver subhaloid. It is therefore assumed that in 

 the case of röntgenography the intermittent exposure, apart from the 

 photographic induction to be exceeded each time, does not produce 

 a photographic effect that remains below that of a continuous 

 irradiation. 



i) Americ. Journ. of Science 1887 ; Vol. 33 ; p. 363. 

 2) Forlschritte a. d. Geb. d. R.-Str. 1901; Bd. V; S. 89. 

 3Y Fertschr. a. d. Gob. d. R -Sfr. 1902, Bd . V, Heft 4. 

 '*) Zeitschr. f. wiss. Phot. 1904; Bd. 1 ; S. 382. 



