148 Methods of Observing Micro-organisms 



The best way to examine living micro-organisms is in what is 

 called the hanging drop. A hollow-ground slide is used, and with 

 the aid of a small camel's-hair pencil a ring of vaselin is drawn on the 

 slide about, not in, the concavity. A drop of the material to be 

 examined is placed in the center of a large clean cover-glass and 

 then placed upon the slide so that the drop hangs in the concavity, 

 but does not touch the glass. The micro-organisms are thus her- 

 metically sealed in an air chamber, and appear under almost the 

 same conditions as in the culture. Such a specimen may be k^t 

 and examined from day to day, the bacteria continuing to live until 

 the oxygen or nutriment is exhausted. By means of a special ap- 

 paratus in which the microscope is placed, the growing bacteria 

 may be watched at any temperature, and exact observations made. 



The hanging drop should always be examined at the edge, as the 

 center is too thick. 



In such a specimen it is possible to determine the shape, size, 



Fig. 25. — The "hanging drop" seen from above and in profile. 



grouping, division, sporulation, and motility of the organism under 

 observation. 



Care should be exercised to use a rather small drop, especially for 

 the detection of motility, as a large one vibrates and masks the 

 motility of the sluggish forms. 



When the bacteria to be observed are in solid or semi-solid culture, 

 a small quantity of the culture should be mixed in a drop of sterile 

 bouillon or other iiuid. 



For observing the growth of bacteria where it is desirable to 

 prevent movement, Hill* has invented an ingenious device which he 

 calls the "hanging block" His directions for preparing it are as 

 follows: 



" Pour melted nutrient agar into a Petri dish to the depth of about one-eighth 

 or one-quarter inch. Cool this agar, and cut from it a block about one-quarter 

 inch to one-third inch square and of the thickness of the agar layer in the dish. 

 This block has a smooth upper and under surface. Place it, under side down, on 

 a slide and protect it from dust. Prepare an emulsion, in sterile water, of tie 

 organism to be examined if it has been grown on a solid medium, or use a broth 

 culture; spread the emulsion or broth upon the upper surface of the block as 



♦"Journal of Medical Research," March, 1502, vol. vii, No. 2; new series, 

 vol. II, 



