MICROSCOPES. 



23 



aberration. The focal distances of the lenses mounted in the single 

 microscopes, are usually l^-inch, 1-inch, ^, ^, l-16th, l-20th of an inch. 



15. Bat the most perfect form of single microscope is that which 

 is represented in the annexed diagram, and which, as far as its power 

 extends, is suitable for 

 the examination of 

 any kind of object. 

 It consists of a cir- 

 cular brass stand, ca- 

 pable of being fixed 

 in every possible in- 

 clination by the ball C 

 attached to the clamp 

 B, by which the whole 

 may be firmly fixed to 

 a board or table ; 

 the circular mirror D 

 working in the arm 

 M is attached to the 

 tube X in the usual 

 way. The whole slides 

 upon the stem, that 

 it may be placed 

 nearer to or farther 

 from the object, ac- 

 cording to the inten- 

 sity of the light re- 

 quired for the various 

 objects under exami- 

 nation. The back of 

 this reflector is flat 

 and polished, so that a 

 monochromatic light 

 reflected from the 

 brass may be employed 



when necessary. 



tig. a. 



