218 PHOTO-MICROGRAPHY [CH. VIII 



For photographing transparent or translucent objects there is a further diffi- 

 culty introduced, viz, greater or less transparency. Assuming that all the rays of 

 the spectrum are equally active, there would still be difficulty because the blue 

 and violet stains used in microscopy are liable to be more transparent than those 

 stained with red and orange, consequently a blue stained preparation is liable to 

 lack in contrast since the light reaching the plate from the object and from the 

 space around it produce so nearly equal effects on the plate. 



On the other hand a red stain gives too much contrast because the light pass- 

 ing through it has little effect as compared with the light going through the space 

 surrounding the object. So far in the discussion it is assumed that the objects 

 are practically transparent and that pure red and blue are used. As a matter of 

 fact the stain renders the specimen somewhat more opaque so that the specimen 

 would be darker than the background in either case. This is especially true of 

 hematoxylin. While hematoxylin is blue or purple, it renders the tissues more or 

 less opaque, so that with a petroleum lamp and isochromatic plates, freshly stained 

 hematoxylin specimens are very easy to get good pictures of. 



\ 356. Color Screens. — As a general statement all color screens which have 

 proved really useful in photographing transparent or translucent microscopic 

 specimens cut off most of the blue end of the spectrum. Others cut off also the 

 red end, leaving only the middle, visually brightest part of the spectrum free. As 

 modern achromatic objectives are corrected for the visual rays, a screen cutting 

 off the blue end of the spectrum serves to obviate any lack of sharpness due to 

 the aberration of the blue rays in such objectives. There is the further advan- 

 tage that with red and yellow objects, the color being in general of the wave 

 length transmitted by the screen would be in true relative shade or contrast be- 

 tween background and object, and give good detail in the object. For the blue 

 object this form of screen is also good for, as it cuts off the blue rays, the effect 

 will be like photographing a gray object where the light and shade depend on the 

 transparency of different parts. The denser the part the more opaque it is and 

 therefore the darker it appears with transmitted light. The background allowing 

 all the colored light to pass is lighter than the blue object and therefore there will 

 be good contrast and also good detail in the object. 



§ 357 Composition of Color Screens. — The most successful color screens are 

 solutions held in parallel sided glass vessels. Colored glass, gelatin, collodion, 

 etc., colored with different chemicals are fairly satisfactory, but not so satisfac- 

 tory as the solutions. 



(1) The most generally used and most generally useful screen is a watery 

 solution of dichromate of potash (K 2 Cr 2 7 ). This cuts off the violet, the blue 

 and the bluish green. The amount of light cut out depends upon the density of the 

 solution and the thickness of the stratum through which the light passes. 



(2) Zetlnow' s cupro-chromate color screen allows wave lengths from 0.570^ 

 to 0.550M to pass, that is yellow light. It is made by dissolving in 250 cc. of water, 

 1 1 grams of pure dry cupric nitrate and 1 gram of chromic acid. The thickness of 

 the stratum of liquid should be about 1 centimeter. 



(3) Gifford's color screen. This is highly spoken of both for observation 

 with the microscope (Carpenter-Dallinger) and for use in photography (J. R. M. 

 S. 1S94, p. 164). It is composed of a strong solution of malachit green in water, 

 glycerin, glycerin jelly, etc. Only a thin stratum such as could be mounted be- 



