COLOR PHOTOGRAPHY 653 



The positive film should be left in a dye bath for approximately 10 min. Following 

 a brief rinse, it can be passed into a clearing bath for about 2 min. and finally washed 

 in several changes of water for about 10 min. It is not advisable to wash the positive 

 in running water. The clearing bath is made up as follows: 



Hypo 50 g. 



Sodium chloride 25 g. 



Water 500 cc. 



The assembly of the color positives on a final paper support can be carried out as 

 follows: A gelatin-coated paper, which could be an ordinary fixed-out bromide of 

 smooth surface, is first thoroughly soaked in water and finally passed into a tray 

 containing a weak gelatin solution such as: 



Nelson gelatin 1 oz. 



Water 32 oz. 



The celluloid containing the blue image is also passed into the tray of gelatin and 

 brought into contact with the bromide paper, emulsion to emulsion, and the two 

 withdrawn, avoiding air bells. The two are now thoroughly squeegeed together to 

 expel any adhering air bells and the surplus gelatin. Following this, the two can be 

 placed under pressure for a few minutes and then allowed to dry in open air. 



When completely dry, the celluloid backing of the blue image must be removed by 

 treating it with acetone. This operation is not difficult as it is only necessary to 

 moisten the celluloid backing with acetone, after which the celluloid can be readily 

 scraped off. After wiping the surface off two or three times with a rag soaked in 

 acetone, the surface is ready for transfer of the second image, which may be either the 

 yellow or the red. Transfer of the yellow and red should also be done in a solution 

 of gelatin to assure perfect adhesion between images. 



This process appears complicated, but beautiful results can readily be obtained. 

 It must be born in mind, however, that basic dyes are not so fast to light as acid dyes 

 and therefore prints made with basic dyes should not be unduly exposed to direct 

 sunlight. 



Color Transparencies. — Color transparencies to be viewed by transmitted light 

 or suitable for projection can be obtained by any of the processes so far described. It 

 is important to note, however, that not only must the positive images be printed 

 stronger but the contrast also must be practically doubled as compared with the paper 

 prints. This becomes immediately obvious by considering the fact that in the case 

 of the paper print the light is absorbed by the color layer in passing through, and 

 absorbed again when reflected back by the paper base. In this manner the densitj^ of 

 the image has a double effect. 



Within moderate values of densities the contrast of the photographic image to be 

 viewed by reflected light is approximately twice as much when compared with trans- 

 mitted light. 



Since the color pictures to be viewed as transparencies are ordinarily mounted 

 between glasses, it is often very convenient to develop one color image to one glass and 

 another one to another glass to be mounted face to face with the first one. The third 

 image on a thin celluloid base can then be mounted in register in between. 



If the transparencies are made entirely by the carbro process, the intermediate 

 step of developing the color tissues on a transparent celluloid base for two of the 

 images can be omitted by developing them directly on the final glass plates, which 

 must naturally have a gelatin coating for best results. The third color, preferably 

 the yellow, will be developed on the celluloid support as usual and finally transferred 

 to one or the other of the two images developed on glass. Since the two glass plates 



