HANGING DROP SLIDE 187 



slide" is used (Fig. 137). 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 micro- 

 scope great care is necessary in order to focus on the bac- 

 teria without breaking the cover. To see the organisms 

 distinctly the lower ins diaphragm of the condenser must be 

 nearly closed, so that the light coming through consists mainly 



Fig. 137. — Hanging drop slide. 



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 " Brown- 

 ian 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. 

 Measiu-ement 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. 

 Cell forms and cell groupings may be studied in the same 



