20 



HISTOLOGY OF MEDICINAL PLANTS 



the lines of the stage micrometer are clearly seen. Then adjust 

 the scales until the lines of the stage micrometer are parallel 

 with and directly under the lines of the ocular micrometer. 



Ascertain the number of lines of the stage micrometer covered 

 by the one hundred lines of the ocular micrometer. Then 

 calculate the value of each line of the ocular. This is done in 

 the following manner: 



If the one hundred lines of the ocular cover seventy-five 

 lines of the stage micrometer, then the one hundred lines of 

 the ocular micrometer are equivalent to seventy-five one- 

 hundredths, or three-fourths, of a millimeter. One line of the 

 ocular micrometer will therefore be equivalent to one-hundredth 



Fig. 23. — Micrometer Eye-Piece 



of seventy-five one-hundredths, or .0075 part of a millimeter, 

 and as a micron is the unit for measuring microscopic objects, 

 this being equivalent to one one-thousandth of a millimeter, 

 the value of each line of the ocular will therefore be 7.5 microns. 



With the high-power objective in place, ascertain the value 

 of each line of the ocular. If one hundred lines of the ocular 

 cover only twelve lines of the stage micrometer, then the one 

 hundred lines of the ocular are equivalent to twelve one-hun- 

 dredths of a millimeter, the value of one line being equivalent 

 to one one-hundredth of twelve one-hundredths, or twelve ten- 

 thousandths of a millimeter, or .0012, or 1.2 u. 



It will therefore be seen that objects as small as a thousandth 

 of a millimeter can be accurately measured by the ocular 

 micrometer. 



In making microscopic measurements it is only necessary 



