( 7 ) 



2. If more light is admitted the image partly disappears, and a 

 more or less fogged plate is obtained with partly a normal and 

 partly a polarized copy, which shows great resistance to further 

 development. 



3. If still more light is admitted, the copy is polarized. 

 This last phenomenon I call Sabatier's polarization. 



J. M. Eder l ) credits Sabatier with first observing it, and says 

 that Seely gave the following explanation of it: The beginning of 

 the development takes place at the surface ; by the secondary expo- 

 sure this developing image is copied upon the silver haloid under- 

 neath it, and as this exposure is more powerful than the first, the 

 second image also develops more strongly, and total polarization results. 



From the experiments carried out by me it appeared that the 

 secondary exposure stopped the development of the surface image, 

 for by reflected light it was seen that it did not increase in density 

 any more, while total development of the whole surface might be 

 expected. In order to ascertain to what extent the copying action of 

 the developing surface image is operative in polarization, I effected 

 the secondary exposure, at the advice of P. H. Eykman, on the glass 

 side of the plate. 



The exposed plate was developed for a short lime, and just after 

 the appearance of the image it was, while still in the developer, 

 covered with a piece of opaque, black paper, which w r as everywhere 

 pressed tightly against the emulsion to prevent the formation of air- 

 bells, from which uneven development might arise, and then the 

 glass side was exposed to direct daylight. As the quantity of developer 

 soaked up by the paper was small and the temperature was below 

 the normal one, the plate, to save time, w T as put in the developer 

 again in the dark room, great care being taken to prevent light 

 from reaching the front of the plate. After fixation a polarized copy 

 appeared. 



This shows that the copying of the developing image at most acts 

 a very secondary part in the appearance of polarization. 



The latter can only be ascribed to the further development of the 

 surface image being stopped, and to the reducibleness of the silver 

 haloid underneath it being increased. Consequently here again two 

 images are formed, one under the other : at the top a normal copy 

 of little density, and under it a polarized one of greater density, 

 corresponding to that of the polarized copy in fig. 3 in the case of 

 solarization. 



i) J. M. Eder Handb d. I'hot. 1898; Bd. II; S. 82. 



