77 



times it is difficult to devise experiments Avhich will decide between 

 v^arious theories, all of which have been made as wide and indefinite 

 as possible and consequently of little practical use. A deeper know- 

 ledge of the latent image can only be obtained by further experiments 

 to test the various theories. 



When silver bromide is precipitated by the addition of a soluble 

 silver salt to potassium bromide in the presence of gelatin a colloidal 

 solution is obtained. If the concentration of the silver salt added to 

 the gelatin solution of potassium bromide is strong, the resulting 

 colloidal solution appears blue by transmitted light-^, ^^, ^'. Such 

 emulsions are sensitive to red and even infra-red light. In the 

 ordinary way emulsions are prepared from a dilute solution of silver 

 nitrate, the emulsion thus obtained appears red by transmitted light: 

 On ripening the colour changes to green. We do not know the size 

 of the particles in the blite emulsion, they are probably very smalL 

 The red emulsion contains particles up to about • 1/^ in diameter, 

 the green emulsion contains particles from -S^u, upwards. In the 

 blue and red emulsions the colours are quite well accounted for by 

 the Rayleigh theory of the scattering of light by small particles, but 

 as yet there is no explanation of the cutting off of the red end of the 

 ■ spectrum by the emulsions with particles of about the diameter of 

 a wave-length of light. Keen and Porter have shown that a similar 

 colour change takes place in a suspension of sulphur when the size 

 of particle becomes a Uttle greater than the wave-length of light. 



BiBLIOGBAPHY. 



Preparation of Emulsions. 



1 Abney, Sir William (Piper & Carter, London, 1883). ' Photography with 

 Emulsions. ' 



2 Eder, Handbuch der Photographie, Vol. III. 



' British Journal of Photography, 275, 286. .300 (1917). 



« Liesegang, R. E., Zeit. Phys. Cheni., 75, 374 (1910). ' Uber die Pveifung 

 von Silberhaloidemulsionen.' 



5 Bancroft, Wilder D., Jour. Phys. Chem., 14, 12, 9C, 201, 620 (1910). ' The 

 Photographic Plate.' The Emulsion, Pt. I., II., III., IV. 



These papers give an excellent summary and extracts from the 

 literature of the subject up to 1910. 

 « Luppo-Cramer, Phot. Korr., 44, 572 (1905). 



Theories of Ripening and the Latent Image. 



' ' Photohalides of Silver.' Carey Lea, Sill. Am. Jour., 3. 33, 349, 476 (1887) ; 

 Sill. Am. Jour., 3, 38, 129, 237, 248 (1889). 



* W. Ostwald, Eder's Jahrbuch fur Photographie (1897), 402. 



' R. Luther, Zeit. Phys. Chem., 30, 628 (1899). ' Studien uber umkehrbaro 

 chemische Prozesse.' 



1" R. Luther, Die Chemische Vorgange in dcr Photographie (1899). 



»i Thiel, Zeit. Anorg. Chem., 24, 32 (1900). ' Formation of Mixed Crystals of 

 the Halides of Silver.' 



12 Monkemeyer, Jahr. Min. Beil, 22, 1 (1906). 



" Bellach, iVilhelm Knapp, Halle (1903). ' Struktur der Photographischen 

 Negative.' 



1' Quincke, Eder's Jahrbuch fur Photographie (1903), 3. 



" W. D. Bancroft, Jour. Phys. Chem.. 12, 209, 318, 417 (1908); 13. 1, 181, 

 260. 499, 538 (1909); 14, 292 (1910). 'The Electrochemistry of Light.' 



