10 APPARATUS 



concave depression 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 



In examining a hanging drop first use a low-power objective and, having brought 



FIG. 4. Blood serum coagulating apparatus. 



into focus the margin of the drop as a center line, change to a r/6- or i/8-in. objective. 

 By this procedure a thin layer of fluid is brought under the high dry objective in- 

 stead of the deeper layer in the center of the drop. It is not advisable 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 

 concave 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 

 solution along with the slide. 



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. It's 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. 



Into a test-tube, about 1X7 in., we introduce the special sugar media, then 

 drop down a small test-tube (1/2X3 m O with its open end downward. Insert the 



