Ch. VII] PHOTOGRAPHY WITH COLOR SCREENS 239 



out in shades of black, a yellow color screen absorbing violet-blue 

 and allowing the other colors to pass is used with a plate sensitive 

 to the other colors to be photographed. A picture is thus obtained 

 which shows the violet-blue objects in black and the other details 

 in various shades. 



A contrast color screen does not of course give correct brightness, 

 but the purpose in using it is to bring out in the most striking manner 

 the form of certain structures. The general law is: For contrast 

 effects, use a color screen which absorbs the light transmitted normally 

 by the colored object, but allows the other colors to pass. 



§ 369. Refraction and opacity and color screens. — It should 

 not be forgotten in using color screens and color-sensitive plates that 

 refraction and opacity exert their full effect in producing the final 

 result. The color screen acts only to suppress or lessen certain definite 

 wave lengths. Refraction and opacity tend to suppress all wave 

 lengths in certain hmited borders or definite areas. Hence' any 

 stain like hematoxyUn which tends to make an object more opaque 

 to aU parts of the spectrum will increase the contrast even if no color 

 screen is used. 



§ 370. Lessening contrast. — With some specimens it is necessary 

 to lessen contrast in order to bring out details of structure. One 

 of the striking examples frequently referred to is whalebone. A 

 microscopic section of this has a reddish appearance by transmitted 

 light. If now a blue screen is used with a panchromatic plate the 

 greatest possible contrast is obtained, and the object loses all detail 

 in the photograph. If on the other hand a red screen is used the 

 photograph shows good detail and the general appearance is like that 

 seen by the eye in looking into the microscope. 



The general law is : When the contrast is too great use a color screen 

 of the same color as the object, and of course a plate must be used 

 sensitive to that color. 



§ 371. Use of the microspectroscope in photo-micrography. — If 

 one studies his specimens with the microspectroscope and makes sure 

 exactly what light is transmitted by them, it will be possible to judge 

 with intelligence what plate and what color screen to use to bring 

 out in the most satisfactory manner their structural appearances. 



