THE MICROSCOPE AND MICROSCOPIC METHODS 31 



the lower surface of this objective. Oil of cedar- wood furnishes 

 a medium having nearly the same refractive index as the glass of 

 the lens and the glass on which the object is mounted, and it ob^ 

 viates the dispersion of light which takes place when a layer of 

 air is interposed between the objective and the object, as happens 

 with the ordinary dry lens. 



The microscope should be placed in front of the observer on 

 a firm table. The observer should be able to bring the eye easily 

 over the eye-piece when the tube of the microscope is in vertical 

 position. Daylight should be employed if possible. When arti- 

 ficial illumination is necessary, an ordinary lamp, a Welsbach 

 burner or an incandescent electric light may be used. It is best 

 to modify the artificial light by inserting a sheet of blue glass be- 

 tween the light and the mirror. 



In order to focus upon any object, having first secured a satis- 

 factory illumination with the mirror, it is best, beginning with 

 the low power and using the coarse adjustment for focusing, to 

 bring the objective quite close to the object, and then, with the 

 eye in position, to raise the tube until the object comes into 

 focus. The exact focusing is done with the fine adjustment. 

 The observer should keep both eyes open when using the micro- 

 scope, and should be able to use either eye at will. 



All measurements of microscopic objects are expressed in 

 terms of a micromillimeter. This is one-thousandth of a milli- 

 meter (o.ooi mm.), which is about -^-^^-Q-Q of an inch. This unit 

 is designated as a micron, and is denoted by the Greek letter /*. 

 For example, 5 /* = 0.005 mrn - = 5~oV<r inch. 



The Platinum Wire. The substance under examination 

 is usually placed upon thin slips of glass called cover-glasses. The 

 material is spread over the cover-glass by means of a platinum 

 wire which has been fixed in a glass rod about six inches long. 

 Such a platinum wire is used constantly in doing bacteriolog- 

 ical work. The platinum wire must be stiff enough not to 

 bend too easily, and yet it should not be so large that it will 

 not cool rapidly after heating. A good size for most pur- 



