MICROMETRY — MICROMETRIC MICROSCOPES 
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camera has been placed. Adjust the illumination even more care¬ 
fully than in ordinary drawing, using axial light. Focus sharply, 
and carefully sketch the outline of the object upon drawing 
board or notebook, using a very hard and sharp-pointed pencil. 
The object is now removed and replaced by a stage micrometer, 
the instrument focused and the graduations of the scale traced 
upon the paper, either across the outline of the object or near 
by. The distance from the camera to the paper must be identical 
in each case. The dimensions of the object may thus be ascer¬ 
tained easily by comparison. 
The method of indicating the size of different objects in draw¬ 
ings of microscopical subjects by means of tracings of a stage 
micrometer is always preferable to a tabulation of numerical 
dimensions, since the indication is a graphic one and appeals to 
the eye at once. Moreover, it enables another investigator to 
ascertain any dimensions indicated in the drawings. 
Method 3 . — Measurements obtained by means of oculars con¬ 
taining ruled scales. Oculars of this type are called Micrometer 
Oculars. There are many forms, but all fall into one of three 
groups; (a) those having a fixed scale; {b) those in which the 
entire scale is movable; (c) oculars having movable scales 
actuated by micrometer screws provided with graduated heads 
indicating the magnitude of displacement in fractions of the 
scale divisions. 
Group b possess few advantages over Group a. Micrometer 
oculars of Group c are generally called Filar Micrometers and 
comprise the most accurate as well as the most convenient 
microscopic measuring devices now in use. 
Since in micrometer oculars the graduated scale is so placed 
as to fall in the same plane as that of the real image formed by 
the microscope, the number of scale graduations covered by the 
image gives a value for the size of the image only and not for the 
object. It is therefore necessary in all cases ^ to first ascertain 
the true value of the eyepiece scale with respect to each 
1 An exception to this statement is to be found in ocular micrometers with scales 
so ruled by the manufacturer as to yield a definite value with objectives supplied 
for use with them. 
