i>7 '•!' OPTICS. 



these advantages, that tlie object appears most 

 clear, they lie in little room, may be carried about 

 any where, are to be had at a small price, and are 

 most easy to be used. 



A very convenient form of microscope is where 

 A B (Plate 14. fig. l.) is a circular piece of wood, 

 ivory, &c. in the middle of which is a small hole, 

 one-twentieth of an inch diameter : upon this hole 

 is fixed, with a wire, a small lens C, whose focal 

 distance is C D. At that distance is a pair of 

 pliers, D E, which may be adjusted by means of 

 the sliding screw, as in the figure, and opened by 

 means of the two little studs ae ; with these you 

 take up any small object, O, and view it with the 

 eye placed in the other focus of the lens at F; and 

 according to the focal length of the lens, the ob- 

 ject O will appear more or less magnified, as repre- 

 sented at I M. If the focal length be half or 

 one-fourth of an inch, the length, surface, and 

 bulk of the object will be magnified as before de- 

 scribed. This small instrument may be put into a 

 case for the pocket. Those lenses, whose focal 

 lengths are three-tenths, four-tenths, and five- 

 tenths of aninch, are the best for common use. 



Since the nearer the eye can approach to an 

 object, the larger it appears, it is plain, a double 

 and equally convex lens magnifies more than a 

 plano-convex lens ; because, if the sphere, or con- 

 vexity, be the same, the focal length of the former 

 is but half as long as of the latter ; and since the 

 double convex consists of two segments of a sphere, 

 the more an object is to be magnified, the greater 

 must the convexity be, and therefore the smaller 

 the sphere ; till at last the utmost degree of mag- 

 nifying will require that these segments become 

 hemispheres, and, consequently, the lens will be 



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