THE MICROSCOPE AND MICROSCOPIC METHODS 3 1 



wood is placed between the lower surface of the objective and 

 the upper surface of the glass covering the object under ex- 

 amination. The oil must be wiped away from the surface of 

 the objective when the examination is finished. For this purpose 

 the soft paper sold by dealers in microscopical apparatus serves 

 admirably. Care must be taken not to scratch 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 obviates 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 well 

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

 tween the light and the mirror or to employ a special incandescent 

 nitrogen bulb made of "daylight" glass, which can be obtained 

 from dealers in electric lamps. 



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 -ob- 

 server should keep both eyes open when using the microscope, 

 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 Hsooo of an inch. This unit 

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

 For example, S/i = 0.005 ™i^- = 3'^ooo inch. 



