HANGING DROP SLIDE. It is sometimes necessary to examine living bacteria and for this purpose the device known as the “hanging drop slide” is used (Fig. 139). The slide has a slight concave depression ground in the middle of one face. A ring of vaseline is placed around this depression with the loop needle. On a clean cover-glass, large enough to fit over the ring of vaseline, several drops of a broth culture, or of material from a solid culture suspended in broth or physiological normal salt solution are placed. The slide is inverted on the cover-glass in such a way that the ring of vaseline seals the latter to the slide. When the whole preparation is quickly turned cover side up, the drops are seen “hanging” to the under side of the cover over the depression in the slide. In examining such a preparation with the microscope great care is necessary in order to focus on the bacteria, without breaking the cover. To see the organisms distinctly the _lower iris diaphragm of the condenser must be nearly closed_, so that the light coming through consists mainly of parallel vertical rays, otherwise the transparent bacteria themselves refract and diffract the light and appear blurred and indistinct. By studying living bacteria with this device it can be determined whether they are motile or not. The motility should not be confounded with the familiar “Brownian movement” of all minute insoluble inert particles which non-motile living bacteria and also dead bacteria show. The hanging drop slide is of value in the measurement of bacteria, since this is properly done on the living organism. Measurement is done with a calibrated ocular micrometer as in other kinds of measurement with the microscope with which the student is presumably familiar. The direct effect of various agents on living bacteria as light, electricity, heat, etc., in the study of “tropisms” and “taxes” has been investigated on various modifications of the above-described hanging drop slide. [Illustration: FIG. 139.--Hanging drop slide.] Cell forms and cell groupings may be studied in the same way but these features are best determined on _stained_ preparations in many instances. “Dark field” illumination and the ultramicroscope are of great value in the study of living bacteria and other minute objects, but apparatus of this type would scarcely be used by the student in an introductory course, so that they will not be discussed in the present volume. STAINING.