216 PHOTO-MICROGRAPHY [CH. VIII 



transmit certain wave lengths of light and absorb others. If the color is a 

 pure orange for example all the other colors of the spectrum have been absorbed 

 by the object ($ \ 214-217). Usually, however a greater or less number of waves 

 of other colors are transmitted also. 



If one wishes to get the greatest possible contrast in photographing an 

 object stained with pure orange a color screen is used transmitting all the other 

 colors except orange. Then as the object can transmit only orange light it 

 absorbs all the light sent to it while on all sides of the object light of all the 

 other wave lengths will reach the photographic plate and affect it, hence in 

 the photograph the orange object appears black in a light field. 



Although objects seen in the microscope may appear of a certain color 

 they usually transmit also wave lengths of other colors so that there is a cer- 

 tain amount of detail shown in the picture due to the different amounts of 

 effective light waves which are transmitted in different parts depending upon 

 the varying density of the object. 



Where there is no detail as with many bacteria, the blacker the object 

 appears in the picture the better, hence in such cases a monochromatic color 

 screen complementary to the color transmitted by the bacteria or other objects 

 would give the most satisfactory results. 



Proper choice of a color filter is greatly aided by studying the object to 

 be photographed with a micro-spectroscope to see what wave lengths and the 

 proportion of each are actually transmitted by the specimen. Then if one 

 studies the color-screens available he will be able to select the one most nearly 

 complementary to the object to be photographed. As stated above, it is desir- 

 able in histologic preparations with structural detail to show such detail. 

 This is partly determined by the different refractive index of the different 

 parts, and it can be greatly accentuated by selecting a color screen which 

 eliminates the excess of the short waves from the light. For many objects 

 stained with dyes giving strong contrast etc. as hematoxylin and carmine, 

 good pictures may be obtained without a color screen if isochromatic plates 

 are employed and a kerosene lamp is used for illumination. The kerosene 

 light is very rich in the waves near the middle of the spectrum, but rather 

 poor in the short waves. 



{! 293. Composition and Preparation of Color Screens. — In recent years 

 as the principles for the proper selection and use of color screens have become 

 more fully understood the range has been greatly increased of the appropriate 

 color-sensitive plates. While color screens of solutions are still used, perhaps 

 the majority of screens now employed are made of variously colored glas9 or 

 of glass coated with variously stained gelatin or collodion. 



By recalling the work with the spectroscope (jj 217), it will be remem- 

 bered that the light transmitted through a colored object depends upon the 

 thickness of the object and also upon the intensity of the illumination. This 

 being true the same color screen may be made to give greater or less contrast 

 in the photograph by varying the intensity of the illumination. If one studies 

 the spectrum of solutions of the various dyes used in microscopy, like 

 aurantia, methyl green, etc., he can select colors for his color screen which 

 give contrast for the specimens he has to photograph, remembering always 



