( s ) 



The similarity between Sabatier's polarization and solarization is 

 so great, that R. Lyle 1 ) attributed the phenomenon mentioned sub 2 

 to the first zero condition of Janssen's periodicities of solarization being 

 reached 2 ). In one of my experiments it appeared to me that this 

 similarity only existed in so far as no image was to be observed. 

 The first zero condition is characterised by the maximum of density 

 obtainable, whilst the plate in question remained very thin. Conse- 

 quently the cause of the disappearance of the image cannot lie in this. 



By transmitted light, too, the plate does not show polarization in 

 the portions exposed most intensively by the primary exposure, 

 but in the portions that received the smallest amount of light. 

 (Therefore Sabatier's polarization cannot be ascribed to solarization). 

 It is easy to understand that the relative liminal value of a lower 

 layer is first reached in those portions where the absorption of the 

 surface image is the least, and where this layer, at the same time, 

 is situated nearest to the free surface. So in this case the copying 

 quality of the surface image exercises its influence. 



IV. HerscheVs effect. 



By Herschel's effect I understand polarization by double exposure. 

 It differs from polarization by solarization in that much smaller 

 amounts of light-energy are sufficient to produce it, and in the 

 reducibleness of the primarily exposed silver haloid decreasing at 

 once on a secondary exposure. 



The duration of the primary exposure must always exceed the 

 liminal value of the plate. After the secondary exposure has exceeded 

 a certain maximum {the critical exposure), the plate shows a normal 

 copy again. The value of the critical exposure depends entirely on 

 the primary exposure. This has led to the so-called Clayden's effect 3 ) 

 (black lightning) being looked upon as a new phenomenon of the 

 photographic plate. 



The first observation dates from 1839, and was made by J. Herschel, 

 who stated that the red and the yellow rays of the spectrum could 



i) Phot. Gentralbl. 1902 ; S. 146. 



2 ) Gompt. rend. 1880: T. 90; p. 1447, T. 91; p. 199. 

 Moniteur de la Phot. 1880; p. 114. 



Beibl. z. d. Annal. d. Phys. u. Ghem. 1880; S. 615. 



J. M. Eder, Handb. d. Phot. 1906; Bd. 1; T. 2; S. 306 1898; Bd. II; S. 78. 



J. M. Eder, Jahrb. f. Phot. u. Repr. 1894; S. 378. 



3) J. M. Eder. Jahrb. f. Phot. u. Repr. 1901; S. 610. 

 Camera Obscura. 1901; bldz. 513. 



J. M. Eder. Handb. d. Phot. 1906; Bd. I; T. 2; S. 312. 

 „ „ 1903; Bd. Ill; S. 834. 



