8i4 BOTANY AND PHARMACOGNOSY. 



scopic study of vegetable drugs a knowledge of the comparative 

 size of the elements is often of much help in determining the 

 identity of material under examination, and for this reason the 

 student should early learn to measure the characteristic elem,ents, 

 or those showing a variation in size in different plants, as starch 

 grains, calcium oxalate crystals, diameter of cells, thickness of 

 cell-walls, etc. The method best adapted for this work is that 

 involving the use of a micrometric scale which is placed in the 

 eye-piece and known as the ocular micrometer. But to determine 

 the value of the ocular micrometer it is necessary to use another 

 scale known as the stage micrometer. The stage micrometer, as 

 its name indicates, is used on the stage, and when placed in juxta- 

 position to an object indicates its size. However, it is obviously 

 impracticable always to place an object along side of the scale, 

 and hence in practice the ocular micrometer is used, the value' 

 of the divisions of which are determined by comparison with 

 those of the stage micrometer (Fig. 328). The value of the 

 divisions of the ocular scale varies for different objectives, eye- 

 pieces and tube lengths, hence it is necessary to ascertain the value 

 of the divisions for the different optical combinations and tube 

 lengths employed. The stage micrometer is usually divided into 

 tenths and hundredths of a millimeter, and the millimeter being 

 equivalent to 1000 microns {[x), the smaller divisions are equiva- 

 lent to 10 microns (10 /x). For example, we may suppose, using 

 a low-power objective, that 10 divisions of the ocular scale equal 

 20 of the smaller divisions of the stage microm,eter. Thus, 20 

 divisions of the stage micrometer are equivalent to 20 times 10 fi, 

 or 200 /a; then, since 10 divisions of the ocular scale equal 20 

 divisions of the stage micrometer, one division of the ocular scale 

 is equivalent to i/io of 200 fi, or 20 fi. Or, using the high-power 

 objective, we may suppose that 80 divisions of the ocular scale 

 equal 24 divisions of the stage micrometer. Thus, i division of 

 the ocular micrometer is equivalent to 1/80 of 240 /x, or 3 fx. 

 Then, if an object has a diameter covering 3 divisions of the 

 ocular micrometer, its diameter is equivalent to 3 times 3 fx (the 

 value of one division), or 9 fx. 



The MiCRO-POLARiscoPE is a useful accessory in conjunction 

 with the microscope. It is useful in the study of technical 



