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 division 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. 



