MICROMETRY — MICROMETRIC MICROSCOPES 
195 
specifications for standard sieves. As most firms have now 
accepted these standards, there is little need, therefore, of 
checking up the wire cloth for ordinary work, but in fine work 
it is always good policy to check the diameter of the wires, 
the number of meshes to the inch, and the area and uniformity 
of the openings. The problem is quite simple when dealing 
with the unmounted fabric but is difficult indeed when the 
sieves themselves must be checked and we have only an ordi¬ 
nary chemical microscope of the small-stage type. The distance 
from the center of the stage (optic axis) to the supporting pillar 
is too small to permit a fair-sized sieve to be examined save 
for an area near the rim. With large-stage microscopes or 
instruments of the Greenough type shown in Fig. 30 no dif¬ 
ficulty will be experienced save with sieves of abnormally great 
diameter. 
Strong surface illumination is essential. The Silverman illu¬ 
minator gives especially excellent results, but a powerful beam 
of light, from a good Mazda lamp and a condensing lens, thrown 
upon the fabric as nearly vertically as possible (or a vertical 
illuminator) will answer all purposes. Use a micrometer eye¬ 
piece, focus the scale with more than ordinary care so that the 
scale divisions stand out very black and distinct. This is essen¬ 
tial since the objects to be measured are opaque. 
Focus sharply upon a wire in the plane of its diameter; bring 
a di\dsion of the micrometer scale in contact with an edge of 
the image, count the number of divisions covered by the image 
and compute the results in the usual manner; make not less 
than three readings before passing to another wire. In each 
case re-focus before counting the scale divisions. In like manner 
measure a number of different wires throughout the area of the 
fabric. Record separately the diameters of warp and shoot 
wires. Warp wires are generally more bent than the shoot wires. 
The number of meshes per linear inch can be best determined 
in coarse sieves by means of an accurately divided rule and for 
medium fine sieves a rule and hand lens is convenient. The 
compound microscope should be used only with very fine wire 
cloth. 
