METHOD OF USING THE MICROMETER. 
53 
in the stage micrometer, and we find the draw-tube extended 
only to inches. Thus, in covering the object with thin 
glass, and turning forward the adjustment of the object-glass 
twelve and one-half degrees, we have found it necessary to push 
in the draw-tube jVo inch, or yf □ inch for every degree the 
adjustment of the object-glass has been turned forward. 
The same method is to be pursued in estimating the value of 
the micrometer lines for every object-glass with which it is to 
be used. The values here given apply only to the particular 
instrument and glasses used in this calculation, for slight differ- 
ences in glasses of the same name, and different lengths of the 
compound body, however slight, cause the measurements of the 
micrometer to vary. Hence every microscope should have its 
own table of measurements for its micrometer. 
72. Method of using the Micrometer. Suppose we are 
examining a delicate Diatomacea with the instrument which 
we have used in making the preceding calculations, and we 
wish to measure the breadth of the rows of hexagonal mark- 
ings which we find covering the object. We examine the 
adjustment of the object-glass, and find that it stands at eleven 
degrees, then yf ^ inch for each degree would give y^^ of an 
inch, which the present adjustment requires to be subtracted 
from yW» the distance the draw-tube is to be extended for an 
uncovered object, which leaves yo”^ of an inch as the distance 
the draw-tube is to be extended to have our micrometer lines 
measure ten-thousandths of an inch ; adjusting the draw-tube 
to this calculation, we see that two spaces in the micrometer 
cover nine rows of the delicate hexagonal markings on our 
object, {navicula angulata^) therefore the breadth of each row 
is Tsioo of an inch. 
Suppose any object measured is equal to a certain number of 
spaces and a small part of an additional space, one-half, or even 
a quarter of a space between two micrometer lines can gen- 
erally be estimated with tolerable accuracy, so that the meas- 
urements made by the glass micrometers attached to a nega- 
tive eye-piece, may be relied upon for measurements as small 
as one twenty thousandth, or even one forty thousandth of an 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
