48 



PRACTICAL TREATISE ON THE MICROSCOPE. 



of natural history should be without one ; its aid is hourly 

 required. There are two forms of these pocket magnifiers in 

 general use ; the most common form, represented by fig. 23, 



carries one, two, or three mag- 

 nifiers, whilst a much larger and 

 more convenient form is repre- 

 sented by fig. 24, in which there 

 are two sets of lenses, varying 

 in their focal length from two 

 inches to a quarter of an inch; 

 between the lenses may be seen 

 ^'S-^^- in both figures the diaphragm 



or stop, which enables us to use the two lenses as a doublet. 



Fig. 24. 



A square hole is made in the end of the handle of fig. 23, and 

 a round one in the middle of that of fig. 24, for the pur- 

 pose of attaching them to a stand, as wiU be subsequently 

 shown. Mr. Smith generally puts three lenses into one 

 handle, the highest power is a planoconvex, the next a crossed 

 lens, and the lowest a double convex lens ; these, when com- 

 bined, perform uncommonly weU. 



When a higher magnifying power is required, the form 

 generally used is that known as the Coddington lens, con- 

 sisting of a sphere of glass, around the equator of which a 

 triangular groove has been cut, and the groove itself subse- 

 quently fiUed up with opaque matter, as represented in section 

 by fig. 25. The great advantage of this form of lens is, that 

 however obliquely pencils of light, B A, may fall upon it, they, 

 like the central ones, pass at right angles with the surface, 



