20 



MANUAL or BACTERIOLOGY. 



as with the objective. A i-inch and ij-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 

 using the xV immersion objective a layer, of thickened oil of 

 cedarwood 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 microscopic ap- 

 paratus serves admirably. Care must be taken not to scratch 

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



Fig. 4. — Abb^ condenser. 

 On the right side the figure gives a sectional view. 



a medium having nearly the same refractive index as the glass 

 of the lens as 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 object is used in 

 connection with the Abbe condenser, which consists of two 

 or three lens 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 less, and substituting the concave for the plane mirror. 



