POSITION OF OBSERVER. 281 



and the tone of the field will be the same as that of the 

 band. The student should practice until he shall be 

 competent to recognize the band and its accompanying 

 tint according to the particular glass employed. This 

 done, let him throw the glass a little out of adjustment, 

 and it will be seen that the band has disappeared, and 

 with it so has all the nice definition pertaining to the 

 edge of the diatom. Readjust the glass and watch for 

 its return as you manipulate. There will be no time 

 wasted at this. 



A remark of a general nature is in place here. In 

 working over shells as coarse as the Cherry field or the 

 rhomboides, if the glass be well adjusted for one valve 

 it may be accepted that it is sufficiently so for all on the 

 mount. This is neither strictly nor practically true. 

 As some of the diatoms lie nearer the cover than others, 

 the adjustment requires to be changed in accordance 

 with the thickness of the intervening strata of -balsam ; 

 and on fine work, like the medium frustules of the 

 Leipsig, this point must be kept well in mind. 



Now, with this point in mind, let the student, return- 

 ing to the Leipsig, adjusting his glass over the same 

 shell we have had in mind, getting things just right " to 

 a dot," let him slide the object carrier and hunt up an- 

 other and similar valve; that is to say, one that lays 

 just as flat in the balsam, and of the same size. Now 

 focus, and endeavor to say positively whether the new 

 shell is the same depth in the balsam as was the former. 

 If it shall be the same depth, that edge band will be 

 seen; otherwise not. Practice this we??, hunting from 



