314 
METHODS IN PLANT HISTOLOGY 
Measuring with the Ocular Micrometer.—A stage micrometer 
and an ocular micrometer are necessary. A stage micrometer should 
be ruled in tenths and one-hundredths of a millimeter. It does not 
matter what the spacing in the ocular micrometer may be, except 
that the lines must be at equal 
distances from one another. 
As a matter of fact, the ocular 
micrometer is generally ruled 
in tenths of a millimeter, but 
this ruling is more'or less mag¬ 
nified by the lens of the ocular. 
Place the stage micrometer 
upon the stage and the ocular 
micrometer in the tube, and 
arrange the two sets of rulings 
so that ‘the first line in the 
ocular micrometer will coincide 
with the first line of the stage 
micrometer, and then find the 
value of one space in the ocular 
micrometer. The method of 
finding this value is shown in 
the following case in which the 
tube length was 160 mm., the 
ocular a Zeiss ocular micro¬ 
meter 2, and the objective a 
Leitz 3. In the ocular micro¬ 
meter, ninety-eight spaces 
covered just fifteen of the larger 
spaces of the stage micrometer. 
Since the stage micrometer 
is ruled in tenths and one- 
hundredths of a millimeter, the 
fifteen spaces equal 1.5 mm., or 
1,500 /i. 1 Then ninety-eight spaces of the ocular micrometer equal 
1,500 ju; and one space in the ocular equals -fa of 1,500 /jl, or 15.3 /*• 
This value being determined, there is no further use for the stage 
micrometer. To measure the diameter of a pollen grain put the 
1 One millimeter = 1,000 p. The Greek letter p is an abbreviation for pinp6v, or micron. 
