EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 3! 



eye-piece (Zeiss No. 2 and No. 4) serve well for most pur- 

 poses. The eye-pieces are usually named arbitrarily, like 

 the objectives. The oil-immersion objective is used in the 

 examination of bacteria where a very high power is desired. 

 A layer of thickened oil of cedar-wood is placed between the 

 lower surface of the objective and the upper surface of the 

 glass covering the object under examination. 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 admi- 

 rably. Care must be taken not to scratch the lower surface 

 of this objective. Oil of cedar-wood furnishes a medium 



FIG. 4. 



Abbe Condenser. On the right side the figure gives a sectional view. 



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. This ob- 

 jective is used in connection with the Abbe condenser, 

 which consists of two or three lenses combined so as to 

 focus the rays coming from the plane mirror upon the 

 object. The condenser gives a very intense illumination 

 over a very small field. The condenser is not necessary 

 excepting with the oil-immersion objective. If it is used 

 with the other objectives the illumination must be regulated 

 by lowering the condenser, closing the diaphragm more or 



