PHOTOGRAPHING BACTERIA. 103 



and in general it will be found more satisfactory to fix the micro- 

 organisms to be photographed to a slide or cover glass by desiccation 

 and heat, and to stain them with one of the aniline colors. 



Objects which are opaque cannot be photographed by transmitted 

 light, and objects which have a deep orange or red color are practi- 

 cally opaque for the actinic rays which are at the violet end of the 

 spectrum. Such objects simply intercept the light, but this gives 

 the outlines, and, where there are no details of structure, is all that 

 is required to illustrate the form and mode of grouping. Softer and 

 more satisfactory photomicrographs of bacteria are made when the 

 staining is not such as to entirely arrest the actinic rays. Among 

 the aniline colors Bismarck brown and vesuvin are the most suitable, 

 care being taken, with the larger bacteria especially, not to make 

 the staining too intense. Objects which are transparent for the ac- 

 tinic rays, or nearly so, give a very feeble photographic image, or 

 none at all, on account of the want of contrast in the impression 

 made upon the sensitive plate. This is the case when we attempt to 

 photograph, by ordinary white light, objects which are stained violet 

 or blue. But this want of contrast in the negative can be overcome 

 by the use of specially prepared plates and colored screens of glass 

 interposed between the object and the source of light. The so-called 

 orthochromatic plates are more sensitive to the rays toward the red 

 end of the spectrum than ordinary plates. They are prepared by 

 treating the plates with a solution of eosin, of erythrosin, or of rose 

 bengal (Vogel), and may now be purchased in this country from 

 dealers in dry plates. If we shut off the violet rays by the use of a 

 yellow screen, objects having a yellow or orange color may be pho- 

 tographed upon orthochromatic plates, although the time of exposure 

 will be quite long owing to the comparatively feeble actinic power 

 of the yellow rays. 



We may also make photomicrographs of objects stained with 

 methylene blue or with fuchsin, because objects stained with these 

 colors are opaque for the rays from the red end of the spectrum, and 

 sufficiently so with yellow light to give a good photographic con- 

 trast. Frankel and Pfeiffer recommend the use of a green light-fil- 

 ter (green glass screen) for all preparations stained with methyl vio- 

 let, fuchsin, or methylene blue; and for brown-stained preparations a 

 pure blue light. The writer has been in the habit of using a yellow 

 glass screen for fuchsin-stained preparations, and has had excellent 

 results, but the time of exposure is necessarily long. A yellow glass 

 screen may be prepared by dissolving tropaeolin in negative varnish, 

 and pouring this upon a clean glass slide, where it is permitted to 

 <lry. 



To show bacteria in photographs in a satisfactory manner we 



