58 



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 ; in doing this grasp the ends of the mirror- 

 fork between the thumb and middle finger and move 

 the mirror by the first finger. If the field can not be 

 evenly illuminated, it is evidence that the mirror is 

 beyond the limit of angular aperture in the objective, 

 and it must therefore be brought back until it is. 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. By means of the micrometer screw carry 

 the fine adjustment back and forth beyond the plane 

 of the object and observe closely whether any lines 

 can be distinguished. It is very probable that they 

 will show ; but if not, the cause should be deter- 

 mined. It may be that the magnifying power is not 

 sufficiently great, and in this case a higher power eye- 

 piece should be used, or the cover glass may be more 

 or less than the normal thickness, which would cause 

 a spherical over or under-correction in the objective. 

 In this case th'e lines would appear when the diatom 

 is not in focus. If the objective is a non-adjustable 

 one, the proper correction may be approximately 

 reached by means of the draw-tube. If the lines 

 appear over the plane of the object, it shows over- 

 correction, and the length of tube should then be 

 decreased, or contrary when the lines show below or 



