PHOTOGRAPHING BACTERIA. 105 
and in general it will be found more satisfactory to fix the micro- 
érganisms 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 tropzolin in negative varnish, 
and pouring this upon a clean glass slide, where it is permitted to 
dry. 
To show bacteria in photographs in a satisfactory manner we 
