138 



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 by the slide P. angn- 

 latuni. 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 position into an oblique one, on the side 

 opposite to that of the light and readjust the 

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

 tween the thumb and middle finger and move the 

 mirror by 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 objec- 

 tive and it must therefore be brought back until 

 the light appears equally well over all parts of the 

 field. It must here also be noticed that if the 

 diaphragm is still attached to the instrument and 

 does not swing with the mirror, it may also be the 

 means of cutting off light. The largest opening 

 should be used or in the case of the cap dia- 



