Gradation of a Developed Photographic Image. 317 



were not the same. To ascertain the cause of this an exposure was 

 made through the slit without any disc intervening, and on develop- 



Fig. 



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Fig. 8. 













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Apertures of Sectors in degrees. 



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ment it was found that the reduction of silver was greatest in that 

 part which was illuminated by the light coming through the edge of 

 the prijsm, and least where it passed through the bases of the prism, 



7 2 



