22 CONSTRUCTION OF 



distinctly seen by them, such as animalcules in water, mites in 

 cheese, hairs of animals, down of moths, etc. ; and the process of chrys- 

 tallization may also be conveniently watched with them. 



13. CODDINGTON'S LENS. Of these simple lenses, the best 

 unquestionably for viewing opaque objects, chrystals, etc. is that 

 which has, from its inventor, been denominated the Coddington lens. 

 It is a small sphere of glass, with its equatorial parts ground away 

 that it may at the same time magnify and yet be corrected for spherical 

 aberration. These lenses are sold by all optical instrument makers. 

 In external form they resemble the Stanhope lens. 



14. A frequent and most convenient 

 form of simple microscope, is that which is 

 represented in the annexed woodcut, and 

 which has the advantage of great porta- 

 bility. From the arrangement of the mount- 

 ings to suit the nature of the objects to be 

 examined, these microscopes often derive 

 their name, though the principle of all 

 of them is similar as botanical, minera- 

 logical, anatomical, natural history, aquatic 

 microscopes. A is a piece of thick brass, 

 with a channel cut along it to enable B to 

 slide along it. The arm A being jointed, is 

 capable of lying flat when out of use, 

 or can be altered to suit any other 

 convenient position. B is a brass pillar terminated by a pair of 

 nippers C, for holding the objects to be examined ; D is a brass 

 socket and screw holding the lens, which may be thus changed for 

 another, if it be desirable ; E is the handle to hold the instru- 

 ment, which may be unscrewed at the lower half, and thus rendered 

 exceedingly compact. There are often in the socket D two lenses, 

 one at each side. These two lenses, when low power is wanted, as 

 in the inspection of flowers, are frequently two plano-convex lenses 

 with their plain sides turned towards the eye. The advantages 

 of this combination are, an increase of field and the diminution of 



Fig. 7. 



