POSITION OF OBSERVER. 249 



be detected. This is not the case with other objects. 

 Show the tyro a scale of podurae under a tolerably nar- 

 row angled one-tenth, and likewise display the same 

 with a similar object glass of the higher balsam angles. 

 There will be as much difference in the quality of the 

 two exhibits as there is (I was going to say) between 

 light and total darkness; and yet, nevertheless, the nov- 

 ice will be as well satisfied with the one as with the 

 other ; but let the experiment be repeated, using in the 

 place of the podura a balsam mount of surriella gemma, 

 both glasses doing their best, as before, and the tyro, is 

 no longer " at sea " as to his choice. Moreover, at this 

 stage of his experience he will be fully prepared to blow 

 his trumpet in the support of one of the most absurd 

 and stupid errors that has ever been promulgated since 

 the time of Adam, to wit, " High angled glasses are only 

 fit for work over diatoms! !" 



When we say that diatoms are the most convenient 

 objects over which to study the adjustment of the ob- 

 jective, we mean it, and thereto attaches greater force 

 than the casual reader may suppose. If it be imagined 

 that these objects are " convenient " because their gen- 

 eral proportions are about the thing because they can 

 be purchased at slight expense, or, if preferred, pre- 

 pared by the observer himself, or even be it granted 

 that the markings on the more difficult of these shells 

 will only surrender to a first-class objective in perfect 

 adjustment, we admit the facts, but the story is not 

 fully told. 



The grand, the culminating convenience attending 



