POSITION OF OBSERVER. 275 



your past experience to bear. Without attempting to 

 observe as to the striee, reckon up the general appear- 

 ances and form some verdict as to whether the glass is 

 in adjustment. If you decide that it is not, make up 

 your mind to what extent. This done, proceed, if ne- 

 cessary, to adjust, following the directions already given, 

 bearing now in mind that all the phenomena previously 

 described will be much fainter and less decided than was 

 the case of the Cherryfield or the Monmouth. 



Let the student assure himself that the shell before 

 him lays perfectly flat in the balsam. If this is not the 

 case, it will be impossible to see both the upper and 

 under edges simultaneously. These frustules are ex- 

 tremely thin, and when one edge is the lower of the 

 two in the mounting medium, it will appear to vanish 

 out of sight. Accepting that the diatom is a favorable 

 one for study, the learner will notice that the upper and 

 lower edges, as the adjustment approaches the proper 

 position, behave better than was the case with the Cher- 

 ryfield or the Monmouth. This is owing, of course, to 

 the diaphanous character of the Leipsic. He will there- 

 fore endeavor to render these edges considerably sharper 

 and cleaner than could possibly have been the case when 

 working over the former slides. Let the pupil observe 

 closely the behavior of the shadow under the median 

 line. As has already been remarked, the effects are quite 

 similar, but in a less marked degree. The eye must be 

 educated to the situation. Notice, too, that as the ad- 

 justment becomes more perfect this shadow contracts, 

 assuming the form of a narrow band immediately adja- 



