754 



A TEXT-BOOK OF BOTANY. 



contents, but which in focussing upon them are seen to be optical 

 or microscopical sections of the pores. 



MICROMETRY OR MICROSCOPIC MEASUREMENT. In the micro- 

 scopic study of any substance a knowl- 

 edge of the comparative size of the 

 elements is often of much help in deter- 

 mining the identity of material under 

 examination, and for this reason the 

 student should early learn to measure the 

 characteristic elements, 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 micro- 

 metric 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 indi- 

 cates, is used on the stage, and when 

 placed in juxtaposition to an object indi- 

 cates its size. However, it is obviously 

 impracticable always to place an object 



alongside of the scale, and hence in prac- 



<HUj^ _...* tice the ocular micrometer is used, the 



. 1 value of the divisions of which are 



c?pe'sfen|Thi ?e?ltk,n r of determined by comparison with those of 

 S a Asthe stage micrometer (Fig. 417). The 

 Su^a! value of the divisions of the ocular scale 

 . varies for different objectives, eye-pieces 



ffi > to af rSta l U?j$: and tube lengths, hence it is necessary to 

 on whld? thici"ar e mi : CTom- 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 (the micron being indicated by the 



