32 SELECTION AND USE 



best microscopes for examining bank bills, fine engravings, and 

 similar objects. The general form is shown in Fig. 6. 



Fig. 6. ENGRAVERS' GLASS. 



Linen Provers. These are a very old form of the simple 

 microscope, and being iu very general use, they are manufac- 

 tured in large quantities, while as it is necessary that they 

 should be of good quality they afford an opportunity ot getting 

 a good magnifier at a moderate price. The general . form is 

 shown in Fig. 7, in which the instru- 

 ment appears as in use. The upper 

 plate carries the lens, and the length of 

 the upright is such that when the base 

 is placed upon any flat surface, that 

 surface will be in focus. The base is 

 pierced with a hole one quarter of an 

 inch square, and when placed on a 

 piece of cloth the lens enables us to 

 count the number of threads which oc- 



Fig. 7.-UNEN PROVER. cu l^ that s P ace - Tllis indicates the 

 fineness of the fabric. 



The Stanhope Lens consists of a cylinder or rod of 

 glass, one end of which is rounded so as to form a lens, while 

 the other end is either flat or slightly curved. The distance 



