238 PHOTOGRAPHY WITH COLOR SCREENS [Ch. VII 



yellow, while if green and red are absorbed the transmitted light will 

 appear blue; if violet-blue and green are absorbed the light will appear 

 red, and if violet-blue and red are largely absorbed the remaining 

 light will appear green. 



§ 367. Compensating ray filters. — These are filters or screens 

 which aid the panchromatic photographic plate in giving a black 

 and white picture of colored objects which shall correspond in bright- 

 ness to the different colors as seen by the eye. 



As all photographic plates, even the panchromatic ones, are more 

 sensitive to the violet-blue than to the other colors of the spectrum 

 (fig. 142), the effect of the violet-blue must be reduced, hence yellow 

 screens must be used to do this and compensate for the smaller sen- 

 sitiveness of the plate for the other parts of the spectrum. 



Fortunately the great photographic manufacturers have made a 

 study of the principles of color screens as well as of their plates, and 

 they supply workers with data showing what wave lengths of light 

 their different plates are sensitive to, and the w r ave lengths absorbed 

 wholly or in part by their ray filters. They also give advice from 

 abundant experience as to the proper combination of plate and color 

 screen to get the best effect in photographing a great variety of 

 colored objects. By using this information, and profiting by expe- 

 rience, one can learn to photograph almost any object successfully. 



§ 368. Contrast ray filters. — ■ These are filters or screens by the 

 aid of which strong contrasts in black and white are given to various 

 colored objects or their details. As given in the general statement of 

 the basis for visibility of objects and their details, refraction and 

 opacity are of prime importance for securing sharp outlines. Color 

 images are also of the greatest advantage in differentiating the details 

 of microscopic structure, but as color does not appear in the ordinary 

 photograph the differentiation of colored objects must be secured by 

 producing shades of light and dark up to complete blackness in some 

 cases. For example, in some microscopic specimens important details 

 may be stained violet or blue. To the eye these violet or blue objects 

 stand out with great clearness. In the photograph, on the other hand, 

 without special help from a color screen, they are wholly lost or are 

 so faint that they can hardly be seen. To make such details stand 



