20 



MANUAL OF BACTERIOLOGY. 



as with the objective. A i inch and i^ inch eye-piece (Zeiss 

 No. 2 and No. 4) serve well for most purposes. The eye- 

 pieces are usually named arbitrarily, like the objectives. In us- 

 ing the -Y^ immersion objective 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 exami- 

 nation. 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 microscopic 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 



Fig. 4. — Abbe Condenser. 

 On the right side the figure gives a sectional view. 



glass on which the object is mounted, and it obviates the dis- 

 persion of light which takes place when a layer of air is inter- 

 posed between the objective and the object, as happens with the 

 ordinary dry lens. This objective is used in connection with 

 the Abbe condenser, which consists of two or three lenses com- 

 bined 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 except- 

 ing 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 less, and substitut- 

 ing the concave for the plane mirror. It is to be remembered 



