33 



by a diaphragm, giving a large, fairly flat field. 

 It is especially used in primary botanical work. 



Another is the Linen Tester (Fig. 17). It is 

 made in various sizes, but the ordinary form has a 



Fig. 17. 



lens of 1 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. Its principal purpose is however, as its 

 name indicates, its use in the textile industries for 

 counting the number of threads which appear in 

 the standard opening of \ inch. 



The most simple form of holder is a base to 

 which is fixed a stem or rod upon which the 

 magnifier may be adjusted. Fig. 18 shows one 

 with a white opal glass plate for placing the object 

 upon. In this or other similar forms the advantage 

 of holding the lens is counterbalanced by disad- 



