POSITION OF OBSERVER. 297 



adjustment. For instance, on the "Aberdeen" it is 

 probable that ome specimens of Mtzschia may be found. 

 by hunting for them, and this should be done at once, 

 and the glass corrected as nearly as possible. An ap- 

 proximate adjustment will suffice to reveal the trans- 

 verse striae, and the initial display ultimately improved 

 by force of a little well directed manipulation. It 

 may occur, too, that some of the nitzschia are more 

 difficult than others. This being the case, the stu- 

 dent is to master these successively before attacking 

 the amphipleuras. 



In the practice of all the preceding lessons, and over 

 the test slides, it will be essential that the pupil, after 

 having adjusted his glass as perfectly as may be, note 

 the exact division of the collar graduations. This he 

 should make a memorandum of, so that he can be able, 

 if necessary, to readjust the objective to the mount 

 without loss of time. But this is not all. He will 

 find that as he acquires proficiency by practice, that he 

 will, time after time, change these recorded numbers, 

 and will be inclined, perhaps, to smile at the wildness 

 attending his initial attempts ; and by thus comparing 

 the present with the past, he may be encouraged as to 

 the future. This habit of noting in black and white 

 the best attained adjustment of the objective from day 

 to day should be rigidly persisted in for at least a 

 twelve-month. 



