200 STUDY OF INDIVIDUAL BACTERIA-STAINING 



slide" is used (Fig. 136). 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 physio- 

 logical 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 "hang- 

 ing" 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 iris diaphragm of the condenser must be 

 nearly closed, so that the light coming through consists 



Fig. 136.— Hanging drop slide. 



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 properl}^ done 

 on the living organism. Measurement is done with a cali- 

 brated 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 



