Canadian machinery and metalworking (January-June 1919) . , the operation of the pro-tractor is rapid. In case of particularlydefective threads, exposures of the pro-jection may be made on any photo-graphic developing paper—with an ex-posure time determined experimentally—and a permanent record obtained ofthe condition of the thread. Construction of Lantern.—The selec-tion of the projection lens is must be free from distortion over theportion of the projected shadow employ-ed in making measurements. It is de-sirable also that a short-focus lens beemployed in ord
Canadian machinery and metalworking (January-June 1919) . , the operation of the pro-tractor is rapid. In case of particularlydefective threads, exposures of the pro-jection may be made on any photo-graphic developing paper—with an ex-posure time determined experimentally—and a permanent record obtained ofthe condition of the thread. Construction of Lantern.—The selec-tion of the projection lens is must be free from distortion over theportion of the projected shadow employ-ed in making measurements. It is de-sirable also that a short-focus lens beemployed in order that a reasonablemagnification may be secured with aminimum distance between the lens andthe screen. A lens which satisfies these conditionsvery well has been secured by combininga 32-mm. aperture, 152-mm. focus, tele-scope objective (lens B), with a TessarSeries 11B, 4x5 photographic objective(lens C). These are mounted in a bias.^tube so that their separation can be ad-justed to a point where the distortioneffects arc a minimum. When properly. FIG. 12 1HREE-BALL METHOD OF MEASURING THE EFFEC-1IVE DIAMETER OF A RING THREAD GAUGE. adjusted these lenses B and C have acombined focal length of approximately100 mm., and a magnification of 50 is ob-tained with a screen at in. from thelenses. Another lens which has been used inplace of both lenses B and C, is a Tes-sar Series IC, 75-mm. focus photo-graphic objective. This lens will give amagnification of about 70 with the samescreen distance as with the combinationof lenses previously mentioned. For the prism, it is necessary to secureone which has its three faces accuratelyflat to within about one wave length oflight. The same degree of flatness isrequired in the mirror, and in additionthe mirror must be silvered on the front. Omitting the Prism and Mirror.—Itwill be found that prisms and mirrorsof the required accuracy are expensive,and moreover, difficult to secure. How-ever, convenience may be sacrificed andthe pri
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmachinery, bookyear19
