HANGING DROP II 



CONCAVE SLIDES, FERMENTATION TUBES 



The concave slide is ordinarily used for making hanging-drop prepa- 

 rations for the examination of bacteria as to motility, capsules, size 

 and arrangement. 



To prepare a hanging-drop preparation for the study of motility it is best to place 

 a loopful of the young bouillon culture or a loopful of salt solution into which is then 

 emulsified a small amount of growth from an agar slant, in the center of the cover- 

 glass; now having applied with a brush a ring of vaseline around the concave depres- 

 sion in the slide we apply the slide as a cover to the cover-glass which latter adheres 

 to the ring of vaseline. The completed hanging-drop preparation can now be turned 

 over and placed on the stage of the microscope. 



A substitute which is equally good may be made by spreading a ring or square of 

 vaseline smaller than the cover-glass to be used in the middle of a plain slide. 

 Then putting a loopful of salt solution in the center of the space, and inoculating with 

 the culture to be studied, we finally cover it with a cover-glass, gently pressing the 

 margins down on the vaseline. This gives a preparation for the study of motility or 

 agglutination which does not dry out for hours, and is easier to focus upon than the 

 concave slide hanging-drop preparation. 



Hanging Drop. In examining a hanging drop first use a low- 

 power objective and, having brought into focus the margin of the drop 



FIG. 3. Hanging drop, over hollow ground slide. (Mac Neal.) 



as a center line, change to a J- or J^-inch objective. By this pro- 

 cedure a thin layer of fluid is brought under the high dry objective 

 instead of the deeper layer in the center of the drop. It is not ad- 

 visable to use an immersion objective with a hanging-drop preparation. 



The light should be cut down to a minimum with the iris diaphragm and the con- 

 cave mirror used. When we have finished examining the preparation the cover-glass 

 should be pushed over with the forceps so that a corner projects and we then seize 

 this with the forceps, lift up the cover-glass and drop it into the disinfecting solu- 

 tion along with the slide. 



Fermentation Tubes. The fermentation tube with a bulb and closed 

 arm is expensive, difficult to clean, and is easily broken. It is, however, 

 convenient in the determination of the gas formula of an organism. 

 Its use is described under water analysis. As a substitute in the study 

 of gas production and in water bacteriology, the Durham tube is to be 

 recommended. 



