176 
ELEMENTARY CHEMICAL MICROSCOPY 
with certainty closer than ±0.2 /jl, this value being, as we have 
already seen, the practical limit of the resolving power of the 
compound microscope (see page 7). But when a series of 
measurements are made of the same object the values obtained 
will usually agree among themselves by less than 0.2 /x, and two 
different experienced microscopists may be expected to obtain 
values which will differ by less than this. Ewell ^ believes that 
microscopic measurements may be relied upon as accurate among 
themselves vnthin less than o.i /x or even under exceptionally 
favorable conditions within 0.05 /x. 
The degree of accuracy obtained will obviously be largely 
dependent upon the resolving power of the objective employed. 
Micrometric measurements obtained with moderate magnifi¬ 
cations are much more accurate as a rule than those obtained 
with high powers. 
Method 1 . — The method of direct comparison of object and scale 
is generally impracticable and seldom available where ordinary 
microscopes are employed, since it is next to impossible to have 
object and scale lie in exactly the same plane under the micro¬ 
scope. But in “ micrometer ” or “ traversing ” microscopes the 
principle made use of is substantially that of a direct comparison 
with a micrometer scale. 
Since the chemist-investigator is not infrequently called upon 
to make long series of microscopic measurements of objects or 
to measure the distance between lines in photographs of spectra, 
etc., types of these special micrometric microscopes are shown in 
Figs. 112, 113 and 114. 
For the comparison of lines in small spectra, scale rulings, etc., 
the traversing microscope shown in Fig. 112^ will be found 
accurate and convenient. This instrument consists of two 
microscopes A and B, mounted in fixed positions on a single 
heavy base. The stage S slides to the right and left in grooves; 
it is provided with two sections S^, S^, of which may be 
^ Ewell, J. Roy. Micr. Soc., 1910, 537. Nelson, ibid., 1910, 696. 
2 The comparator illustrated in Fig. 112 is manufactured by Carl Zeiss. For 
methods for determining the corrections to be applied to micrometer microscopes, 
consult Scientific Paper No. 215, U. S. Bureau Standards, by A. W. Gray, Microm- 
ter Microscopes (1913). 
