CHOICE OF OBJECTIVES FOR REGULAR WORK. 205 



rest for the arms can no longer be obtained unless some 

 recourse be had to blocking up by books and the like. 



Now in an examination involving three or four hours' 

 time, it is quite possible that it may be desirable to sub- 

 stitute in the place of the two-thirds a glass that will 

 give higher amplifications. In this case the two-third 

 would be removed'and the one-sixth called on to take its 

 place. Now I have previously found out that the one- 

 sixth has maximum performance when worked over the 

 thickest covers. These I have already selected and 

 placed in a little box by themselves, one of which is 

 carefully cleaned and placed over my object, the micro- 

 scope tube pulled out to the standard length, the instru- 

 ment inclined to a suitable angle, and thus the exam- 

 ination goes on. In extreme cases the one-tenth would 

 be again substituted for the one-sixth. With this glass, 

 having as it has, maximum performance at any point f 

 its cover adjustment, there need be no particular care 

 exercised as to the selection of the cover, further than 

 to see that the same be thin enough. 



It will be noticed that with the employment of the 

 immersions the tube should be restored to the standard 

 length. This is an important item, and should never be 

 omitted when there is nice work in hand. These wide- 

 apertured glasses are especially intended by the optician 

 to be worked with tubes of standard lengths. The 

 range of collar adjustment, too, is in many instances 

 arranged conformably thereto. 



In observations over urinary deposits I contrive to do 

 a great deal of work with the one-half inch. Of the 



