59 



beyond the plane of the object. If the above direc- 

 tions have been followed, the lines cannot fail to be 

 seen with a moderately good j or 4 inch objective ; 

 but if they are not, the trial should be repeated. 

 Again, be careful to have no obstruction 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 position 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 ; longitudinal 

 or transverse in others, according to their position ; 

 anft, if the resolution is very good, these lines will 

 further resolve themselves in minute beads of a hexa- 

 gonal form. 



It will now be well to bring the mirror more nearly 

 to a central position ; do this at intervals of about 10 

 degrees, and note the appearance at each decrease of 

 obliquity. It will be found that as the mirrror 

 approaches the optical axis the lines will appear to 

 become more faint, and may disappear before central 

 illumination is reached ; in this case it will be well to 

 begin again. An endeavor should be made to make 

 each attempt give better results than the preceding 

 one. Repeated trials will not only impress the var- 

 ious phenomena upon the mind, but will cause a nota- 

 ble improvement in manipulative skill, and thus a 

 better performance in the objective. 



Until now we have assumed that transmitted light 

 has been used. We will now suppose that the object 



