12 THE STRUCTURE OF 



of an inch in the distance of the lens from the 

 object. When moved through only one of its 

 divisions we obtain a result equal to the 1500th of 

 an inch, and by causing it to rotate through half a 

 division we secure a movement not exceeding the 

 3000th part of an inch in extent. Such nicety in 

 the adjustment of the optical part of the Micro- 

 scope may seem to the beginner unnecessary, but 

 when he comes to work with high powers he will 

 find that he needs the most delicate mechanical 

 contrivances to enable him to secure the proper 

 focus of a sensitive object-glass. 



But this is not the only use to which we can put the 

 fine adjustment. The same process that serves to re- 

 gulate the focusof a lens will also enable us to measure 

 pretty accurately the thickness of an object or any 

 of the small prominences or depressions found in its 

 structure. By observing the number of divisions 

 through which the head of the screw is made to 

 pass while changing the focus of the object-glass 

 from the bottom to the top of any small cavity or 

 prominence we get a tolerable notion of its depth 

 or height, &c. Connected with this apparatus is a 

 special contrivance for protecting the object-glass 

 to some extent from injury. It will sometimes 

 happen, even with the most careful, when using 

 high powers, that the lens is brought down with 

 aome force in contact with the glass cover that 

 protects the object. This risk is not unfrequcntly 

 incurred by admitting to one's study incautious 

 friends, whose confidence is only equalled by their 

 ignorance; who although they may have never seen a 

 Microscope before, will proceed to turn it up and 

 down with a force sufficient to crack the lens. 

 Such friends would have sufficient confidence in 

 themselves to take the command of a man-of-war, 

 even though it were the first time in their lives 



