57 



In this case the object will be placed out of focus, 

 and another adjustment becomes necessary. If a 

 higher power is desired, the draw-tube may be 

 extended. Observe whether the field is well illumi- 

 nated, and if not, bear in mind what has been said about 

 properly adjusting the mirror. If the object appears 

 milky or the light is so intense as to be painful to the 

 eye, which is of usual occurrence to the beginner, the 

 diaphragm should be turned from one aperture to 

 another until a marked difference is seen ; or, the plane 

 mirror should be used. In this connection it is well to 

 state that the above precautions should always be 

 observed with low powers, unless the object is thick. 

 Now use the micrometer screw and note carefully the 

 beautiful structure which is opened to view. After 

 sufficient time has been spent, upon this, the objec- 

 tive may be replaced by a higher power and the 

 object by a slide of P. angulatum ; focus upon this, 

 being mindful of the suggestions previously given, and 

 do not fail to observe what has been said in regard 

 to a well illuminated field. If lamp light from a flat 

 wick is used, turn the edge of the flame toward the 

 mirror, and use the concave side of the latter. If the 

 diaphragm is in an adjustable sub-stage, bring it as 

 close to the stage as possible, or, whether here or 

 attached to the stage, it may as well be removed for 

 the present. Observe now whether outside of the cen- 

 tral rib any lines can be seen upon the surface of the 

 diatoms ; if not, vary the distance of the mirror from 

 the object; or, if lamp light is used, bring the lamp 



