218 LABORATORY EXERCISES IN BACTERIOLOGY. 



express the extremes of variation. In making measurements one should not be content 

 with but a single observation, but should measure a number of individuals of the 

 preparation (and, too, of the same organism grown under varying conditions), express- 

 ing as the result of the investigation either the range of size or the average size of the 

 individuals. 



For measurement of such objects the same technique is to be employed as in 

 measuring other minute bodies. A reliable stage micrometer is focussed with the 

 various lenses (no oil to be used when employing the oil-immersion lens), and when 

 proper focus is obtained for each, an eyepiece fitted with micrometer substituted for 

 ordinary ocular. The divisions of the eyepiece micrometer are now calculated in 

 relation with the known measurements of the stage micrometer and carefully recorded 

 lest they be forgotten. The stage micrometer is now put aside and the microscopic 

 preparations of the bacterium substituted and brought into clear focus, the length 

 and thickness for at least ten individuals determined by comparison with the rulings 

 in the eyepiece micrometer, and the limits or the average set down as the result. Thus, 

 one would express the measurements of Bacillus typhosus as 0.5-0.8 : 1-3 //, meaning 

 that it is from five-tenths to ight-tenths of a micromillimeter in thickness and from 

 one to three micromillimeters in length (micromillimeter, one thousandth of a milli- 

 meter, one twenty-five-thousandth of an inch). The ordinary bacteria range from 

 somewhat less than one micromillimeter to five or ten micromillimeters in long diameter, 

 but there are organisms much smaller, and others three or four times the larger limit 

 just mentioned. 



Exercise 5 1 . Calculate the value of the spaces in the eyepiece microm- 

 eter for the one-twelfth inch oil-immersion lens, and for the one-seventh 

 inch dry lens. Then determine, as instructed, the size of the bacteria of 

 several of the previously stained preparations (e.g., Micrococcus pyogenes 

 aureus; Bacillus typhosus; Mycobacterium diphtheria; Mycobacterium 

 tuberculosis} . 



5. Motility of Bacteria. One of the most striking phenomena to be noted in 

 examining unstained specimens of bacteria suspended in some fluid, as in a hanging 

 drop, is the motion exhibited by the individual cells of many types. (This power of 

 movement may be so persistent that it may remain for some minutes in a dried but 

 poorly fixed film, even after staining and mounting in balsam.) The movements may 

 be very active, darting or undulating in type; or may be slow and scarcely perceptible. 

 One must not, however, confuse the quivering, dancing movement ("Brownian move- 

 ment") which may appear in case of non-motile bacteria, as well as in case of any 

 inert or dead particles when suspended in a fluid, for true movement; when difficulty 

 is met in distinguishing, one may place the organism in some germicidal solution, as 

 of carbolic acid or corrosive sublimate, when if movement persist it is not due to organic 

 Energy. The passive motion induced by currents under the cover-glass, bearing the 

 bacteria, motile and non-motile, in streams in different directions, must not be mistaken 

 for active motion. The means of these active progressive movements were long sus- 

 pected to be due to the possession of ciliate or flagellate organs before the discovery 

 of such appendages by special methods of demonstration. Even to-day it is impossible 

 from insufficiency of our methods, however, to recognize these organs in a number of 

 motile varieties. In the family of the Beggiatoaceae a peculiar creeping or waving 



