139 



phragm, remove it. 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 determined. It may be that the 

 magnifying power is not sufficiently great and in 

 this case a higher power eyepiece 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 the 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 contrar)^ when the lines show below 

 or beyond the plane of the object. If the above 

 directions have been followed, the lines cannot 

 fail to be seen with a moderately good J- or ^ inch 

 objective ; but if they are not, the trial should be 

 repeated. Again, be careful to have no obstruc- 

 tion between the course of rays from the mirror 

 to the stage ; get good illumination on the object ; 

 observe well ; keep the instrument in such a posi- 

 tion that the object is not illuminated from any 

 other direction than from the mirror. 



When the diatoms are resolved in this manner 

 the lines will appear to be diagonal in some, longi- 



