are two convex lenses, usually having a power of 

 ten diameters, which are separated by a diaphragm, 

 giving a large, fairly flat field. It is especially used in 

 primary botanical and zoological work. 



Another is the Linen Tester, Fig. 21. It is made 

 in various sizes, but the ordinary form has a lens of 

 i inch focus. It is arranged 

 to fold into small compass for 

 convenience in carrying, and 

 when opened for use is placed 

 over the object so that this 

 comes into the square opening 

 in the base and is then exactly 

 in the focus of the lens. Its 

 F . 21 principal use, as its name in- 



dicates, is in the textile indus- 

 tries for counting the number of threads which appear 

 in the standard openings of \ or \ inch. 



The simplest form of holder is a base to which is 

 fixed a series of ball and socket joints, which offer 

 means of adjusting the magnifier in every direction. 

 A spring clip is provided for holding magnifiers of 

 different sizes. The lens is focused by raising and 

 lowering the lens clip by hand. In some of the more 

 complex holders there is a rack and pinion for focusing 

 the lans which adds to convenience in woiking. 



For finer dissecting work a stand with fiim base, 

 stage, lans holder, and mirror, and with adjustment for 



