18 PEACTICAL BOTANY 



should not be smeared with Canada balsam or Dammar : 

 when this has happened the lens should be gently rubbed 

 with a cloth wetted with a very little benzol, or alcohol. 

 In all cases the cleaning of lenses should be carried out 

 as gently as possible, to avoid destroying their polish. 



The best light for microscopic work is that reflected 

 from white clouds in a northern sky, and a window with 

 a northern aspect should be selected. Never use 

 direct sunlight, and avoid using artificial light. If the 

 only available room has a south aspect, a white blind 

 is to be used, so as to cut off direct sunlight, or a piece 

 of white card may be fitted to the surface of the 

 mirror, so as to act as a less perfect reflector. 



The body of the microscope should be vertical ; with the 

 short microscopes now in use, the oblique position is 

 quite unnecessary, and very inconvenient when mount- 

 ing in fluid media, or irrigating with fluid reagents. 



Always examine an object with a loiv pmver first, and 

 afterwards, if necessary, with a higher power. It is a 

 general principle of microscopic practice that observa- 

 tions should be made with the lowest possible power sufficient 

 for distinct vision. Never use the high power unless the 

 oliject be covered with a cover-slip. 



When a low power is used a larger hole of the diaphragm 

 below the stage is to be placed opposite the aperture in 

 the stage ; when a high power is used a smaller hole of 

 the diaphragm is necessary, otherwise the definition will 

 not be satisfactory. 



Some difficulty will be felt at first in finding the 

 focus. There are two adjustments of focus the coarse and 

 the fine : the latter is never to be used until the focus 

 is approximately found with the coarse adjustment. 



