152 HOW TO SEE WITH THE MICROSCOPE. 



rience you have gained with the use of the one-fourth 

 will be of the utmost value to- you. Moreover, the 

 one-fourth, too, will continue to be a useful interme- 

 diate glass. 



And thus in replying to all my correspondents, I 

 recognize the importance that one and all shall early 

 become acquainted with the manipulations of adjusting 

 glasses. To accomplish this they must use objectives 

 of a reliable class, i. e., such as will respond promptly 

 to change of collar-adjustment, keeping well in mind 

 the importance of buying nothing to be discarded or 

 thrown out of use in the future. In the case under 

 consideration, it will happen, in nine cases out of ten, 

 that in less than two years the buyer will feel that he 

 needs a first-class inch, or perhaps a two-thirds, in 

 which event the old inch will be of great service as a 

 sub-stage condenser, providing that the stand has been 

 selected with this end in view. 



It may further happen, in truth it will be likely to 

 happen, that in the course of one or two years, my 

 correspondent will either push his own observations or 

 desire to keep pace with those of others, and over 

 structures of the most delicate and " difficult" charac- 

 ter, and now he will need a one-sixth or a one-tenth of 

 the widest possible aperture. Allowing this to occur 

 he will have expended but $110, which is less than the 

 usual cost of a nominally first-class one-sixteenth, and 

 all the glasses on hand will still be of service. Besides 

 all this, he is well armed and equipped for any work 

 requiring powers from 50 to 5,000 diameters, and there 



