276 ^^^ Microscope. [June, 



consist of six or seven lenses, fitted to each other, and if the curves 

 of their surface vary the least from a certain calculated form, the 

 lens is inferior or worthless. When the reader is informed that 

 these lenses are sometimes so minute as to require a magnifier to see 

 them well, he will appreciate the difficulty of grinding, polishing, 

 and accurately centering six or seven such little pieces as constitute 

 an object glass ; nor will he wonder that from sixty to one hundred 

 and twenty five dollars is demanded for an object glass alone. 



The most valuable improvement in this instrument has been 

 made within the last twenty-five years, and so perfect is a first class 

 instrument now constructed, that theory can scarcely require greater 

 perfection than practice furnishes. 



As the optical part approaches perfection, the mechanical adjuncts 

 and mountings must also keep pace with it or they will be valueless ; 

 hence all the adjustments and movements, require the utmost refine- 

 ment of mechanical skill. 



In using the higher powers, the tube carrying the glasses must be 

 set precisely at a certain distance (as found on trial) from the object. 

 And should it vary the one ten-thousandth of an inch from this dis- 

 tance, the view is destroyed. To bring the instrument readily to the 

 proper place, two adjustments are furnished; the large milled head 

 on the side of the tube as seen on the engraving moves a rack and 

 pinion, furnishing what is called the coarse adjustment, by which 

 the tube is rapidly moved to near the focus ; it is then accurately 

 adjusted by means of the milled head near the bottom of the tube; 

 this is attached to a very fine screw, which moves a lever joined to 

 the tube holding the object glass, so that the least possible variation 

 of distance is hereby obtained. This is also used to measure the 

 thickness of very minute objects ; for instance, a hair, or fiber of 

 wool is under examination ; we adjust the focus to the upper side of 

 the hair, which is readily ascertained by its distinct appearance when 

 in focus, while the parts farthest from the glass appear dim and in- 

 distinct. We now turn the milled head of the fine adjustment until 

 the farther side of the hair is in plain view. The distance the glass 

 is moved, as registered by the graduated head, gives the true diam- 

 eter of the hair. 



The fact that objects of any thickness can not have all parts dis- 

 tinctly seen at the same time, is often a source of great disappoint- 

 ment to the observer who is not familiar with the laws of optics. — 

 He supposes that anything he may thrust before the glass will be 



