182 
ELEMENTARY CHEMICAL MICROSCOPY 
objective used. This is accomplished by means of a stage microm¬ 
eter. 
Focus the eye lens of the ocular so that the graduations of the 
ocular scale become clear and distinct. Lay the stage microm¬ 
eter upon the stage and move it until the center of the rulings 
falls in the optic axis of the microscope, focus carefully and 
adjust the micrometers by turning ocular or stage or both until 
the rulings in one scale are parallel to those in the other. Move 
the stage micrometer until a line becomes coincident with a line 
of the ocular scale. Count the number of divisions of the ocular 
scale included between one or more divisions of the stage microm¬ 
eter. Divide the value of the stage scale by the number just 
obtained. The quotient equals the true value of one ocular 
scale division. It is usually the case that conditions obtain 
giving an appearance shown in Fig. 115. It is obvious that in 
such an event it is nec¬ 
essary to estimate with 
the eye what fractional 
part of a division to 
add to the whole num¬ 
ber of divisions of the 
ocular scale included in 
one division of the 
stage micrometer. Such 
an estimation or guess 
introduces a serious 
error into our method. 
Moreover, the image of 
an object to be meas¬ 
ured rarely covers ex¬ 
actly a whole number 
of divisions of the ocular micrometer and we are obliged to 
make a guess as to what fraction of a part to add. Thus there 
are two estimates necessary and any measurements recorded 
must necessarily be mere approximations. The second of these 
errors cannot be eliminated in micrometer oculars with fixed 
scales having rulings of non-variable magnitude, but the deter- 
Fig. 115. Micrometer Scales Improperly Adjusted. 
