4 ELEMENTARY CHEMICAL MICROSCOPY 
making up the object may be raised or lowered in the mount¬ 
ing, thus affording a correction for the displacement of the image 
brought about by the cover-glass. By consulting the diagram, 
Fig. i8,page 40, it will be seen that by turning the collar C the 
combination of lenses L will be displaced and their distance 
from the combination L' will either be increased or diminished. 
A spiral spring S holds the movable parts firmly in place. A 
cover-glass which is thicker than that for which the objective 
is corrected affects the image in the same manner as if the spheri¬ 
cal aberration were over-corrected, while on the other hand if 
too thin the effect produced is similar to that of under-correc¬ 
tion. In the first case the focal distance of the objective must 
be increased, and in the second, decreased. This is accomplished 
by turning the adjusting collar to the right or left, as the case 
may require, or, in the absence of such a de\dce, by shortening 
or lengthening the distance between the eyepiece and the objec¬ 
tive, shortening for cover-glasses too thick, and lengthening for 
those which are too thin. Fitting into the body tube of modern 
microscopes is a tube which may be drawn out several centi¬ 
meters. This tube is known as the draw-tube and is graduated 
in millimeters. Objectives are commonly corrected (for use on 
the usual type of microscope) for a tube length of 160 milli¬ 
meters.^ The 160-millimeter mark will therefore be found only 
when the draw-tube is pulled out a short distance. This position 
of the standard mark permits lengthening or shortening the 
draw-tube, and thus correcting for cover-glass thickness as stated 
above. 
In addition to corrections for chromatic and spherical aberra¬ 
tion at least two other factors must be taken into account in 
comparing, or choosing between, objectives of similar equivalent 
focal length. These are the angular aperture and the numerical 
aperture of the objectives. By the angular aperture of an objec¬ 
tive is meant the “ angle contained, in each case, between the 
most diverging rays issuing from the axial point of an object 
(i.e., a point in the object situated on the optic axis of the micro- 
^ Most metallographic microscopes, however, require objectives corrected for 
200 mm. tubes and are designed to be employed without cover-glasses. 
