8 THE AMERICAN MONTHLY [January, 



The Colored Screen in Photo-Micrography. 



[abstract.] 

 By Prof. ROMYN HITCHCOCK. 



An ordinary gelatino-bromide plate is sensitive to the spectrum of 

 sunlight from a point between the Frauenhofer lines E and F to about 

 K. The maximum photographic action is about G. Bv considerably 

 prolonging the time of exposure the limit of photographic action at the 

 red end of the spectrum is greatly extended. In practice the light be- 

 lo\v the green of the spectrum may be regarded as quite inactive when 

 we take photographs with ordinary plates. 



By introducing a colored screen, a plate of yellow glass for example, 

 in the path of the light, we may absorb the more active rays, and pro- 

 long the time of exposure until the yellow rays have time to act upon 

 the sensitive plate. In practice, however, it is found that there are two 

 difficulties about this method of procedure ; first, in obtaining a satis- 

 factory screen, and second, in the long exposure necessary when work- 

 ing with the comparatively inactive rays. 



With color-sensitive plates, such as are now in general use abroad 

 and gradually being introduced in this country, the range of photo- 

 graphic action towards the red is greatly extended. With such plates 

 the yellow screen can be used with great advantage. 



A few years since it was customary to work with monochromatic 

 blue light in photo-micrography, and the ammonio-sulphate of copper 

 blue cell was much in use. When color-sensitive plates were intro- 

 duced yellow screens took the place of blue, because it was found that 

 many specimens had yellow and red and brown parts which were not 

 well photographed with blue light. 



The color and thickness of the screen both require attention. If it 

 be too thin the blue light is not sufficiently cut off. In particular cases 

 an almost monochromatic yellow light is desirable, as when it is de- 

 sired to obtain sharp outlines of deeply stained objects regardless of 

 structural details. But generally a rather broader spectrum range is 

 desirable, for the light employed should correspond to the difierent 

 colors or shades of color of the object. It is owing to neglect of this 

 consideration that we often see photo-micrographs which are mere 

 silhouettes, while the objects show much more structure to the eye. 

 This is frequently observed in photographs of such structures as the 

 tongue and sting of a bee and legs of insects. In other preparations in 

 which the color is a stain — brown or red for example — the fault lies 

 partly in the exposure, which, in many cases, is insufficient to give 

 more than outlines and blank interiors. This is frequently noticeable 

 in photographs of bacteria. 



By a proper choice of a screen, if a screen is required, a photograph 

 should show any object as clearly as we can see it in the microscope. 



Color-sensitive plates may be said to be indispensable in the photog- 

 raphy of rock sections with polarized light. 



The yellow solution devised by Prof. Zettnow, of Berlin, is used 

 with much favor by many workers. It is composed as follows : Copper 

 sulphate, 175 grammes ; potassic bichromate, 17 grammes ; water, 1,000 

 c.c. 



The true function of the color screen should be to give definition and 

 detail, not to increase contrast betv^een the object and the field, as many 

 observers seem to believe. 



