272 HOW TO SEE WITH THE MICROSCOPE. 



will, and for the reason named, be advantageous before 

 studying the tone of the field. These last two lessons 

 should not only be well studied, but the student ought 

 constantly to endeavor to improve in his recognition of 

 tone. Once having become tolerably, I may say, com- 

 fortably expert, its practice becomes a second nature 

 and will be persisted* in from choice, meanwhile the 

 observer becoming daily more and more proficient. 



This characteristic tone of the field is more apparent 

 in objectives of the highest balsam angles, with the 

 super-excellent object-glasses of the Messrs. Spencer or 

 Mr. Tolles, when the glass is exactly in correction, the 

 tone of the field becomes a peculiar and exceedingly 

 delicate shade of apple green, which one soon learns to 

 recognize by aid of the teacher. 



In the studies we have thus far presented, and which 

 ought to enlist the occasional attention of the pupil for 

 at least a month or six weeks, it is taken for granted 

 that he will meanwhile use the instrument as he may 

 elect, and either for pleasure or such profit as he can 

 arrive at on his own account. There will undoubtedly 

 be other objects brought into requisition not named in 

 our lists. Now, whatever practice he may have of this 

 description, there are some general conditions concern- 

 ing high-angled objectives which ought not to be un- 

 heeded, for example, let the learner discover as near as 

 possible the point ot the maximum aperture of his 

 glass ; it will be well, too, that he pay attention as to 

 thickness of the covers used. These two points have a 

 direct bearing on the pupil's progress and must not be 

 neglected. 



