110 PRACTICAL PHOTO-MICBOGRAPHY. 



duces differentiation in the body-color of the insect but a 

 yellow screen which prevents the background from being over- 

 exposed. It must always be remembered in dealing with 

 orthochromatics that we cannot always make practice agree 

 with theory, and the only way to succeed with some objects is 

 to " ring the changes " till we do succeed. 



Without color-correct photography it would be found well- 

 nigh impossible to photograph any thin section or very minute 

 object stained lightly with that very useful dye, gentian violet. 

 The same remark applies to another favorite stain, methyl blue. 

 Gentian violet used with an orthochromatic plate in presence 

 of a yellow screen, or illuminated by an oil lamp, is, in our 

 opinion, the most satisfactory stain for nearly all bacteria, for, 

 in fact, all that are amenable to the violet stain. And methyl 

 blue is a very useful contrast stain for such micro-organisms 

 as we choose to stain red. If a preparation be stained with a 

 clear violet so as to show body-details in the object, or if a 

 preparation is double stained, red and blue, then a color-correct 

 plate, generally helped by yellow screen or yellow light, will 

 give renderings quite beyond the reach of an ordinary plate. 

 This we have verified a hundred times. 



"Test" diatoms are sometimes mounted in media having 

 very high refraction indices but of very yellow color. We 

 refer to certain arsenic compounds used for the mounting of 

 Amphipleura pellucida and such like. Over and above a dif- 

 ficulty entailed by the optical principles involved when we are 

 working at very wide numerical apertures, this yellowness of 

 the mounting medium makes it very difficult to get a white 

 background in our print. A yellow-sensitive plate used with- 

 out a screen frequently lessens the difficulty alluded to in con- 

 nection with the background. 



In concluding this chapter we can only say that while we 

 apologize for the meagre amount of definite instruction given 

 by us under this head, it must be remembered that the vari- 

 eties of color arid shade are almost infinite ; that unfortunately 

 language is not sufficiently accurate to permit of accurate color 

 nomenclature ; and lastly, that experience alone can teach us 

 how best to choose a suitable plate and a suitable screen for 

 each subject as it falls to our lot to photograph it. 



