PREPARATION Oh' XP]<:CLMJ:\- . 385 



so-called nose-piece. This is a revolving objective holder, by 

 which the objectives may easily be changed. The nose-piece 

 may be arranged for the reception of two to live lenses 

 usually three. 



Finally, some practical points in the use of the microscope 

 may be mentioned : 



(a) The lenses and mirror should be cleaned only with fine, 

 soft cloths. Xylol and other fluids which dissolve Canada 

 balsam should not be used. 



(b) The best light is obtained from a sky covered with 

 white clouds. Direct sunlight is usually not good. Artificial 

 light never gives as good results as indirect sunlight. The 

 Auer light and the incandescent electric light are the best 

 artificial sources, if used with a blue glass over the diaphragm. 



(c) Strong objectives need a strong light. With weak mag- 

 nifications the flat mirror can be used ; with higher powers, 

 the concave mirror ; and with the highest enlargement a con- 

 denser is necessary. 



(d) With strong magnification the diaphragm may be 

 enlarged ; with low powers a small opening is used. 



(e) The focal distance of weak lenses is great. The stronger 

 the lens the shorter the focal distance. 



(/) Every object should be examined first with weak mag- 

 nification, and afterward with higher powers. 



(g] Objectives with weak oculars give clearer pictures than 

 with strong ones. It is better to use a strong objective with a 

 weak ocular than a weak objective with a strong ocular. 



(h) By means of the micrometer screw different levels of 

 the same preparation may be studied. 



2. THE PREPARATION OF SPECIMENS FOR MICROSCOPIC STUDY. 



The elements of the animal organism may be investigated, 

 either in the living condition, or after having been prepared 

 by special methods. 



Only a few things can be studied under the microscope with- 

 out preparation. Among these, may be mentioned fluids such 

 as blood, urine, spermatic fluid, etc. A drop is placed on a 



