654 HANDBOOK OF PHOTOGRAPHY 



will be ultimately bound together, emulsion to emulsion, one of the two images devel- 

 oped on glass must be reversed with respect to the other. 



It is recommended that for the yellow image a specially transparent yellow tissue 

 be used instead of the standard yellow ordinarily employed for paper prints. 



Other alternative arrangements suggest themselves. For instance, the blue-green 

 image could be obtained by iron toning, and the magenta and yellow either by dye 

 transfer or by the carbro method. A suitable formula for producing very acceptable 

 blue-green images by an iron toning is the following: 



Ammonium persulphate 1 g. 



Ferric alum (ferric ammonium sulphate) 2.5 g. 



Oxalic acid 6 g. 



Potassium ferricyanide 2 g. 



Ammonium alum 10 g. 



Hydrochloric acid (10 per cent) 2 cc. 



Water to make 2000 cc. 



The method of eompoimding this bath is rather important. Each chemical should 

 be dissolved separately in a small quantity of warm water, allowing it to cool and then 

 filtering it into a tank in the order given above — with the water added to make the 

 required volume. A properlj^ compounded bath should be pale yellow and perfectly 

 clear. The ordinary toning time is about 10 min. Since this toning bath has a strong 

 intensifying action, the original positive must be rather thin. 



It is not, in general, recommended, however, to make transparencies by combining 

 various processes because it is usually difficult to obtain an over-all satisfactory con- 

 trast except by very careful study of the chemical action of each method 

 employed. 



Additive Processes. — The additive process which permits color reproductions to 

 be obtained bj^ the addition of color lights has found extensive applications for color 

 transparencies. As far as the color prints on paper are concerned, however, it has not 

 met with any degree of success, although several attempts have been made from time 

 to time. 



The chief drawback of any additive process is the great waste of light encountered 

 in viewing the final color image. This point becomes immediately apparent if we 

 consider the additive synthesis obtained by means of three black-and-white positives 

 made from the original three color-separation negatives, these positives being pro- 

 jected by illuminating each one of them with light of the proper color. 



When a single source of light is available, it simply means that the source of light 

 must be split up in three components and into each component the appropriate pro- 

 jection filter introduced. If it is assumed, for instance, that each filter will transmit 

 only one-third of the white light directed on it, then the total light available on the 

 screen will be approximatelj^ one-third of the light from the original source. In prac- 

 tice this efficiency is never reached, and ordinarily the total amount of light available 

 will be nearer to one-fourth of the total light emitted by the source or even less. 



Although theoretically both additive and subtractive processes are strictly iden- 

 tical, in practice the additive process is capable of permitting far more faithful results 

 than the subtractive process if the waste of light is not taken into consideration. This, 

 however, is not always the case, particularly in connection with motion pictures where 

 the amount of light available must be utilized with the greatest possible efficiency. In 

 such case the tendency is to reach a compromise between quality of results and 

 efficiency. This is done by utilizing very diluted projection filters and very thin 

 positives. When a suitable compromise is reached, it is ordinarily found that the 

 results are very comparable with the subtractive process or not as good. Since 

 mechanical difficulties are introduced by the requirement imposed by the additive 



