8 HANDBOOK OF PHOTOGRAPHY 



from the green-filter negative is projected through the green filter; and, finally, the 

 blue-violet- filter positive is projected through the blue- violet filter. 



Since the red-filter positive will be transparent where there was red in the subject, 

 the red beam of the lantern will get through the positive and be projected upon the 

 screen. Since, however, no green or blue-violet got through the red filter, these parts 

 of the scene will be represented on the red-filter positive by dense deposits of silver, 

 and none of the red lantern beam will get to the screen. Similarly, the green and 

 blue- violet portions of the subject will be projected upon the screen by the appro- 

 priate lanterns. 



Present-day color films and plates (except Kodachrome) are based on the additive 

 principle. Between the support and the photosensitive material is placed a myriad 

 of small blue, red, and green filters arranged in a regular or a heterogeneous pattern. 

 The film or plate is exposed with the support side of the structure next to the lens, 

 so that the rays of light reflected from the subject must go through the support and 

 the myriad of filters. Light from a red portion of the subject will get through the 

 red filters and expose the sensitive grains immediately behind the small red filters. 

 Similarly the blue and green portions of the subject are recorded behind their corre- 

 sponding filters. The material is now developed as a negative. The portions repre- 

 senting color in the original subject are deposits of silver and are more or less opaque. 

 The negative is then bleached which removes these silver deposits and renders the 

 negative transparent in the regions where there was light of the appropriate color in 

 the subject. Next, the film is exposed to white light and developed again. In this 

 process the silver halides, not exposed, by the reflected light from the subject and 

 representing portions of the subject which reflected no light, are rendered opaque. 



If, therefore, there were no filters in the material, the plate or film would look like 

 any other positive transparency. The small filters, however, are not destroyed by 

 the processing, and light must pass through them before passing through the trans- 

 parent portion of the positive which represent colored portions of the original subject. 



In the subtractive process black-and-white positive prints are made from each 

 of the separation negatives. The opacity of each part of these positives is inversely 

 proportional to the light reflected from portions of the subject in which there was 

 color of the wavelengths transmitted by the filter through which the corresponding 

 negative was made. A heavy silver deposit on the red-filter positive represents a 

 portion of the subject in which there was very little red. Looked at in another way, 

 this heavy silver deposit represents a portion of the subject in which there was con- 

 siderable nonred. Since white light minus red appears blue-green to the eye, the 

 nonred portions of the positive are colored nonred (blue-green.) By any one of 

 several processes, therefore, the silver deposits in the positives are dyed in colors 

 complementary to those of the filters through which the negatives were made. 



Thus the blue-violet- filter positive is colored yellow; the red-filter positive is 

 colored blue-green (cyan); and the green-filter positive is colored magenta. When 

 these three positives are superimposed and viewed by transmitted light or by light 

 reflected from a white support placed under the three superposed positives (paper 

 print), a colored image of the subject appears. 



The essential difference between the additive and the subtractive processes is the 

 use of colored light for viewing the additive positives (either from a lantern in projec- 

 tion or from the multitudinous filters which are part of the color material) and the 

 use of white light for viewing the subtractive prints. In the additive processes the 

 final result is made up by the addition of the individual contributions of the several 

 colored images to an unilluminated screen. In the subtractive process, the purpose 

 of the blue-green positive (made from the red-record negative) is to subtract from 

 the white light, by which the result is viewed, the wavelengths that did not exist in 



