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 
Fic. 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 .cor2, 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 
