274 HOW TO SEE WITH THE MICROSCOPE. 



may be well to shove the lamp a little farther away from 

 you ; i. e., a little more in advance of the front edge of 

 the stage, or perhaps raising or lowering the wick, thus 

 increasing or diminishing the amount of light, may be 

 of service. This is all that can be written, and the 

 learner must simply make the most of and help himself. 

 Rest assured that both sets of lines are there, and 

 if it be that they are not shown, the diatom is not to 

 blame. In case of total failure the observer may select 

 one of the larger shells, proceeding to the smaller one? 

 carefully and by degrees. With the illumination at 

 present employed he cannot expect thus to show both 

 sets on frustules smaller than a medium one. After 

 having mastered, as well as may be, the Monmouth, 

 substitute the Cherryfield, readjusting the objective, as 

 a matter of course. 



There now remain, for future consideration, on the 

 slides of Monmouth and the Cherryfield, the smallest 

 frustules of the rhomboides. These we will pass over 

 for the present, and in the next lesson proceed to attack 

 the Saxonicas, from Leipsic, Germany. 



Lesson Ninth. The student will recognize the frus- 

 tules on this slide by their similarity to the two slides 

 preceding. The valves are on an average smaller and 

 thinner, and in consequence weaker objects to deal with ; 

 as the saying is, they are more " difficult." Select one 

 of the largest and most vigorous shells ; bring to the 

 centre of the field, and, employing the inch ocular, focus, 

 the illumination being about 70 from axis. Now ex- 

 amine closely your diatom, and see if you can bring 



