THREE-COLOUR PHOTOMICROGRAPHY 207 



gelatine unacted upon by light, and still soluble, have 

 been dissolved away. The presence of silver salts in the 

 emulsion renders development easy, as all details of the 

 image can be distinctly seen. To get rid of all traces of 

 silver salt in the emulsion the films are placed in hypo 

 until the creamy appearance entirely disappears and 

 three perfectly transparent positives are obtained. They 

 are then washed thoroughly and dyed, as follows, in 

 solutions complementary in colour to their respective 

 taking screens. 



Film A from the red negative is dyed in a dilute acid 

 solution of fast green. 



Film B from the green negative is dyed in a solution 

 of erythrosine, and afterwards bathed in a 5% solution 

 of copper sulphate, which fixes the dye to a certain ex- 

 tent and renders the colour less liable to fade on exposure 

 to light. 



Film C from the blue-violet negative is dyed in a dilute 

 acidulated solution of napthol yellow. 



After dyeing, and before drying, the three films are 

 placed one above another in correct superposition to see 

 if the colours are of the right tint, as the correct rendering 

 of the colours of the original depends on the correct tint 

 being obtained in each of the coloured transparencies. 

 Any deficiency or excess of one or other of the colours 

 can be made good by further dyeing, or longer soaking 

 of the particular film in water to which a few drops of 

 acetic acid have been added. When a correct appearance 

 has been arrived at the three positives are dried. The 

 red one is then carefully superimposed on the blue one, 

 and the two fastened together. Finally the yellow 

 print is adjusted on the others, and all three bound 

 together between two cover-glasses. 



In dyeing the positives it is advisable to use very 

 dilute solutions of the dyes, and to leave the prints in them 

 for a considerable time, as this tends to give more vigorous 



