is one which the microscopist cannot too soon learn or 

 too constantly practice. Besides these general warn- 

 ings, however, certain special cautions should be given 

 to the young microscopist with regard to errors into 

 which he is liable to be led even when the very best 

 instruments are employed." 



Medium Power Objective. After sufficient 

 time has been devoted to study with the low power 

 objective, exchange it for the higher power and replace 

 the object with the slide Pleurosigma angulatum. 

 Focus upon this, being mindful of the suggestions 

 previously given and do not fail to observe what has 

 been said in regard to well illuminated field. Observe 

 now whether any lines can be seen upon the surface of 

 the diatoms. If not, vary the distance of the mirror from 

 the object, if adjustment is provided for ; or, if lamp- 

 light is used, bring the lamp closer to or remove it from 

 the instrument in one line so that the illumination will 

 not disappear. If this does not bring out the lines, 

 swing the mirror-bar from the central to an oblique 

 position on the side opposite to that of the light and 

 readjust the mirror. Grasp the ends of the mirror-fork 

 between the thumb and middle finger and move the 

 mirror with the first finger. If the field cannot be evenly 

 illuminated, it is evident that the mirror is beyond the 

 limit of angular aperture of the objective and must 

 therefore be brought back until the light appears 



136 



