THE SIMPLE MICROSCOPE. 47 



CHAPTER I. 



THE SIMPLE MICROSCOPE. 



The simple microscopes in general use may be divided 

 into two classes; first, those used in the hand; and, se- 

 condly, those provided with a stand or apparatus for 

 supporting the object to be viewed, together with an ad- 

 justment of the magnifying power to and from that object, 

 vrith a mirror or speculum for reflecting the light through 

 such objects as are transparent, and a condenser for such as 

 are opaque. 



To the first class, or those microscopes used in the hand, 

 belong the various kinds of pocket lenses, or magnifying 

 glasses so commonly used ; they consist for the most part of 

 double convex or planoconvex lenses of glass, varying in focal 

 length from the quarter of an inch to two inches; one or more 

 of these is set in a frame of metal, horn, or tortoisesheU, and 

 is made to shut up between two other plates of the same 

 material, which, besides forming a handle for it, serve to keep 

 it free from dust and scratches ; the shutting up is similar to 

 that of a knife-blade into its handle. Sometimes these lenses 

 are set in pairs, with a thin piece of horn or tortoisesheU 

 between them, having a hole in its centre corresponding to 

 the centre or axis of the two lenses ; this serves as a stop to 

 cut oflT all the outer rays of light, so that when an object is 

 viewed by the combined power of the two lenses, it is not 

 only more magnified, but the defining power of the instrument 

 is increased in a like proportion, so that we might almost call 

 it a doublet. 



These magnifying glasses are extremely useful for all pur- 

 poses where a high power is not required ; to the anatomist 

 they are essential for examining preparations either in or out 

 of bottles, and for dissections and injections. For the latter 

 purpose, a lens of half-an-inch focus wiU magnify sufficiently 

 to enable an observer to pronounce whether the vessels of 

 most tissues be perfectly filled, or whether extravasations have 

 taken place. In short, no person in the pursuit of any branch 



