THE ANGULAR MICROMETER 
pressure upon the vernier-arm. The microscope should be securely 
clamped during the process as it is important that the axis of the 
microscope should not be tilted at the same time. With the ordinary 
close fit of the oculars of standard makes there is no alteration of the 
apparent position of the pointer hair. 
VII. Errors Caused by Tilting the Microscope Axis 
To detect this error, remove the micrometer and close the substage 
iris-diaphragm until it forms a pin-point aperture and substitute for 
the ocular the pin-point cap, provided with the microscope. If the 
eye is now applied to the opening in the cap a beam of light will be 
perceived if the condenser and the lenses are in alignment as they 
should be for any detailed microscopic work. If the condenser is not 
centered, it should be adjusted until a beam of light reaches the eye 
through the pin-hole aperture. The micrometer should now be at- 
tached and the process repeated. If the micrometer tilts the tube the 
beam of light will no longer be perceived and the micrometer should 
be counterpoised. This is usually not necessary if the rack-and- 
pinion coarse adjustment fits as tightly as it should to prevent the 
tube from gradually sinking during the progress of the measurements 
and producing focal errors. 
For obvious reasons the sources of possible inherent error have 
been considered somewhat at length, and although each instrument 
must be examined for error, it must be apparent that, assuming only 
reasonable skill in instrument construction, the number of actual 
errors in any instrument is small and that, moreover, they are all 
easily detected. Since the accuracy of an\^ instrument of precision 
depends not only upon the theory of its construction and the care 
and skill with which this theory is realized in the instrument itself, 
but also upon the care and skill with which it is used, methods for 
avoiding those errors of practice w^hich are generally known to be the 
most important may be briefly stated in conclusion. 
The calibration of the angular m.icrometer, as of all ocalar microm- 
eters, requires the knowledge of the absolute distance between two 
fixed points on a suitable slide. The commercial stage micrometers 
have been found to be almost useless under high magnification, not 
only because of their variation in spacing but also because of the width 
and irregularity of their lines. The following method was accordingly 
