36 GENERAL BACTERIOLOGY 



objective, prevents the dispersion of light, thus permitting the use 

 of lenses having a greater numerical aperture and longer working 

 distance for the same degree of amplification than is possible with 

 the dry system. In using an immersion lens, place a small drop of 

 immersion oil on the preparation, then carefully lower the objective 

 until it touches the oil drop and nearly touches the cover-glass. 

 Apply eye to the ocular and focus upward very slowly with fine ad- 

 justment until the definition is clear. At the close of the day's 

 work the oil must be removed from the objective and cover-glass. 

 This is best accomplished by wiping them with a piece of Japanese 

 paper made for the purpose. In case the oil should accidentally dry 

 on the objective, it can be removed by adding a little more oil and 

 allowing it to stand for a few minutes ; it can then be wiped off 

 with paper. If this method does not succeed, the objective should 

 be taken to the instructor. Great care must be observed since 

 solvents of the oil are also solvents for the lens mountings. 



Repeeences. See Gage ; A. 199 ; H. 118 ; M. & R. 87 ; P. 206. 



Special Directions. Examine cover-glass preparations made in 

 previous exercise, first with ^ in. objective, and then with the oil- 

 immersion objective. If the specimen be satisfactory, sketch as 

 directed in next exercise. 



EXERCISE 20. DRAWING BACTERIA 



General Directions. In drawing bacteria only a few organisms 

 occurring in the microscopic field should be sketched, but these should 

 be made of considerable size so that the exact outline may be indi- 

 cated. 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 micro- 

 meter 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 in- 

 clude 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 ( G age ) . If the result be not in terms of the micron 



