50 



PRACTICAL TREATISE ON THE MICROSCOPE. 



ratus by means of which it may be supported and adjusted, 

 making it, in fact, a single microscope ; and as it would be 

 foreign to our purpose here to enter into the details of the 

 various contrivances which have been adopted, from time to 

 time, we shall merely make mention of those useful in micro- 

 scopical investigations. 



The mo9t simple, but not the least useful of the single 

 microscopes, is represented by fig. 27. It is principally used by 



watchmakers and wood- 

 engravers, and consists 

 of a loaded stand, of 

 metal or wood, from 

 which rises a circular 

 stem of stout wire or 

 tube ; upon this slides 

 another piece of tube, 

 carrying an arm also of 

 stout wire, having at its 

 end a ball and socket 

 joint, and to the ball of 

 this joint is attached a 

 second smaller arm, to 

 the end of which last, is 

 fitted either a spring or else a ring, serving the purpose of 

 carrying the lens; when the spring is used, the magnifier 

 generally employed is the one the watchmaker adapts to his 

 eye, it is represented by fig. 28, and is nothing more than a 

 lens of an inch focus, set in a long cell of 

 horn, enlarged at one end like a trumpet, 

 this enables it to be grasped firmly by the 

 muscles around the orbit, or if the ring be 

 used, the lens may drop into it. The coarse 

 adjustment is made by sliding the tube up 

 or down the stem, whilst a finer adjustment 

 is secured by means of the small arm and 

 the ball and socket joint ; but it will be seen that if this last 

 be used, and the arm be moved into any other position than a 

 horizontal one, the lens wiU not be in a plane at right angles 



Fig. 27. 



Fig. 28. 



