28 : General Bacteriology. 
c. Make similar preparations from the gelatin and potato and note any variations in 
form, size. ete. ? 
d. All these preparations are to be mounted in balsam, sketched (XXIII) and 
handed to instructor for inspection. 
P EXERCISE XXIII. DRAWING BACTERIA. 
GENERAL Directions. In drawing bacteria only a few organisms occuring in the 
microscopic field should be sketched, but these should be made of considerable size so 
that the exact outline may be indicated. Furthermore they should be drawn to scale and 
individuals selected to give range in form and size. ; 
To measure microscopic objects an ocular micrometer is used, and the first step will 
be to determine its value. Place the ocular micrometer on the diaphragm in the ocular, 
use a stage micrometer as an object and focus. The image of the scale on the stage mi- 
crometer will appear imposed on that of the ocular micrometer. Make the lines of the 
two micrometers parallel and then make any two lines of the stage micrometer coincide 
with any two on the ocular micrometer, pulling out the draw-tube if necessary. Divide 
the value of the included space or spaces on the stage micrometer by the number of 
divisions on the ocular micrometer required to include them, and the quotient so obtained 
will give the valuation of the ocular micrometer in fractions of the units of measure of 
the stage micrometer (Gage). If result is notin terms of micron (/) it should be con- 
verted to such, as this is the unit in micrometry. 
In making drawings represent a micron by two millimeters on paper. This will give 
a magnification of 2,000 (x 2,000). 
REFERENCES. G. 100-108. 
SPECIAL DIRECTIONS. 
a. Determine the value of the ocular micrometer and fill out blanks in following 
table: 
No. of Microscope.............0000000. Mak6 sei cwstensesccton. 
Ocular............ iM., OF ........00- mm 
Value of single di- 
Objective. Tube length. vision on scale 
in 4, 
4 in. (16 mm.) 
+ in. (4 mm.) 
Oil-immersion. 
b. Make drawings of cover-glass preparations made in XXII in place provided in 
table. . 
