102 PHOTOGRAPHING BACTERIA. 



Since the above was written considerable progress has been made 

 in removing the technical difficulties, and a few 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 ; 



