498,377. Optical testing - apparatus. KAPELLA, Ltd., WARMISHAM, A., and REASON, R. E. June 2, 1937, Nos. 19536/38 (divided out of 498,382) and 19537/38. [Class 97 (i)] In optical measuring or testing apparatus in which a beam of light bounded at least on one side by a plane or other surface is directed on an object such as a screw-thread or a wheel tooth and the section profile in which the object is cut by the bounding surface is optically projected, the illuminating device includes two or more slits or knife edges or a single shaped slit or knife edge arranged so that non-collinear parts of the section profile are simultaneously and sharply defined. In an arrangement: for illuminating the two flanks of a screw-throad, two illuminated slits or knife edges are arranged parallel to the two flanks and are focused by two wide-angle lenses on the flanks. If the direction of illumination is substantially inclined to the perpendicular to the flank A<1> or A<2>, Fig. 2, a wide-angle lens is not used, and the slits C<3>, C4 are inclined to the flanks, the nodal planes of the objectives C<1>, C<2> passing through the intersection of the slit with the flank. In Fig. 3, a thread A is illuminated by light from a source D<1> and condenser D<2> passing over a knife edge D shaped approximately to the shape of the profile, an image of the knife edge being focused by an optical system D<3>, D<4> , D<5>, arranged as described in Specification 498,419, on the surface of the thread A. The knife edge D may be made by bending sheet metal and cutting it off in a plane. Alternatively, the knife edge may be made by cutting a solid counterpart, e.g. a nut, which mates with the thread, and polishing the cut surface. Figs. 7 to 9 show a device by which the two flanks of a tooth can be simultaneously illuminated. Two knife edges E<1>, E<2>, formed by the sides of a narrow slit in a flat piece of metal E, are illuminated by a source E<3> and condenser E<4>, and are focused by lenses E<5>, E<6> and a reflecting system, on the flanks A<1>, A<2> of a tooth. The reflecting system comprises symmetrical vertical reflectors E', E<10>, and inclined reflectors E<8>, E<11> which direct the light downward on to the surfaces E<9>, E<12>. A mask E<13> comprising a vertical wire, prevents the projection of an image of both knife edges on each tooth face. The line E<15> is imaged at E<14>, so that the plate E may be displaced parallel to itself to adjust the system for teeth of different size. Rotation of the plate about the base of the slot alters the angle between E<9> and E<12> to accommodate teeth of different angle. Fig. 16 shows the mechanism for adjusting the plate E. The rotational adjustment of the plate E is effected by a cam G<1> engaging a roller G<2> on an arm G<3> forming part of a U-shaped member connected by a link G<5> to the plate E. The translation of the plate E is effected by a cam H<1> engaging a roller H<2> carried by a lever H<3> pivoted at H<4> and engaging a pin J on the plate E. The pin J is in line with the base of the slot between the knife edges and acts as a pivot for the plate E when the cam G<1> is rotated. The translation of the plate E does not materially disturb its orientation, since the links provide a parallel motion. The illuminating devices described are combined with a projecting device whereby an image of the section profile on the surface of the object is obliquely projected without distortion. The apparatus may be used for measuring a profile of a screw-thread or worm defined by a section plane through the axis, or of a gear-wheel by a section plane at right-angles to the axis, or inclined in the case of helical gears. The pitch of a thread may be measured by projecting two or more threads and measuring their separation, or by projecting one tooth and moving the thread axially till an image of another tooth occupies the same position, which may be recorded photographically. The second image may be reversed to obtain meshing of the two images by rotating the illuminating device through 180‹, or as shown in Fig. 17. Two light sources K, K<1>, with condensers K<2>, K<3> and knife edges K<4>, K<5> are combined with a single optical system K<6> and a semi-transparent reflector K<7>. Either knife edge is projected according to the position of the shutter K<8>, opposite sides of the section plane being illuminated accordingly. Alternatively, either of two oppositely directed knife edges may be projected by a single light source, according to the position of a sliding double wedge, or other deviating device. The apparatus is applicable to determine whether two co-operating objects such as a worm and worm wheel will mesh properly, by illuminating each object in turn while the other is removed. To avoid interference by reflected light from other parts of the object, the illuminating beam may be polarized, and a crossed analyzer provided in the projecting beam to cut out reflected light which is still polarized, the diffused depolarized light being transmitted. In another modification, slits or knife edges are not imaged on the surface of the object, but a knife edge is arranged in a parallel beam so close to the object that a sharply defined shadow is thrown thereon. Specifications 412,179, 498,378, 498,382, 498,420, and 498,458 also are referred to.