104 PHOTOGRAPHING BACTERIA. 
Since the above was written considerable progress has been made 
in removing the technical difficulties, and many bacteriologists have 
succeeded in making very satisfactory photomicrographs. As speci- 
mens of what may be done with the best apparatus and the highest 
degree of skill, we may call attention to the photomicrographs in 
the Atlas der Bakterienkunde of Frankel and Pfeiffer, and those 
of Roux in the Annales of the Pasteur Institute. The writer, also, 
has devoted much time to making photomicrographs which have 
served as illustrations for several of his published works. 
Those who have had no practical experience in making photo- 
micrographs are apt to expect too much and to underestimate the 
technical difficulties. Objects which under the microscope give a 
beautiful picture, which we desire to reproduce by photography, may 
be entirely unsuited for the purpose. In photographing with high 
powers it is necessary that the objects to be photographed be in a 
single plane and not crowded together or overlying each other. 
For this reason photographing bacteria in sections presents special 
difficulties, and satisfactory results can only be obtained when the 
sections are extremely thin and the bacteria well stained. Even 
with the best preparations of this kind much care must be taken in 
selecting a field for photography. It must be remembered that the 
expert microscopist, in examining a section with high powers, has 
his finger on the fine adjustment screw and focuses up and down to 
bring different planes into view. He is in the habit of fixing his at- 
tention on that part of the field which is in the focus and disregard- 
ing the rest. But in a photograph the part of the field not in focus 
appears in a prominent way which mars the beauty of the picture. 
In a cover-glass preparation made from a pure culture, when the 
bacteria are well distributed, this difficulty does not present itself, as 
the bacteria are all lying in a single plane; but the portion of the field 
which can be shown at one time is limited by the spherical aberra- 
tion of the objective, which the makers do not seem able to overcome 
in high-power lenses of wide angle, at least not without loss of de- 
fining power. 
Usually preparations of bacteria are stained for photography, 
but with some of the larger forms, such as the anthrax bacillus, 
very satisfactory photomicrographs may be made from unstained 
preparations. In this case a small quantity of a recent culture is 
put upon a slide, covered with a thin cover glass, and placed at once 
upon the stage of the microscope. The main difficulty to be encoun- 
tered results from the change of location of the suspended bacteria 
resulting from the pressure of the objective in focussing. Motile 
bacteria, of course, cannot be photographed in this way without first 
arresting their movements by means of some germicidal agent ; 
