28 



General Bacteriology. 



c. Make similar preparations from the gelatin and potato and note any variations in 

 form, size. etc. 



d. All these preparations are to be mounted in balsam, sketched (XXIII) and 

 handed to instructor for inspection. 



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 maybe 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 not in 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: 



6. Make drawings of cover- glass preparations made in XXII in place provided in 

 table. 



