POSITION ~OF OBSERVER. 265 



shell of the rhomboides, place it -in the centre of the 

 field, the focus well adjusted, etc. Devote a few mo- 

 ments to a general consideration of the nature of the 

 changes wrought by the turn of the collar. Endeavor 

 to gather up, as it were, the experience gained by les- 

 son first. This done, removing the eye for a moment, 

 turn the adjustment in an opposite direction to that of 

 lesson first, but not so far in extent, and immediately 

 looking through the tube, adjust again the focus, and if 

 necessary adjust also the mirror until the best view of 

 the shell shall be obtained. 



Now if the preceding lesson has been well studied, the 

 learner ought to recognize a decided improvement, and 

 there ought, withal, to be more general brilliancy to the 

 image. The shell should, so to speak, lay flatter in the 

 field. In short, things ought to assume an encouraging 

 appearance. Repeat the experiment several times, but 

 with greater or lesser changes of the collar adjustment, 

 until you arrive at a particular point in the adjustment 

 which seems to be about the thing. Having thus deter- 

 mined this point, the student may now, by way of en- 

 couragement, shift the position of the object carrier 

 .and bring one of the larger shells to the centre of the 

 field, when, with a slight alteration of the mirror, he 

 will probably be rewarded with a fair view of the trans- 

 verse stria?. Assuming this to be so, let the student 

 -examine progressively other and smaller frustules, and 

 selecting one of the very smallest that he can see, bring 

 it to the centre of the field. 



Lesson Third. Replace the collar to the initial po- 



