853,978. Optical-projection apparatus. VOIGTLANDER A.G. Feb. 22, 1957 [Feb. 24, 1956], No. 6090/57. Class 97(1) [Also in Groups XXII and XXIII] A grinding machine is provided with optical apparatus for the projection of an image of the workpiece 106, Fig. 8, and, by a path which includes reflection, the image of a template 101 of the tool 105, or a part of the tool itself, the arrangement being such that the image of the template can be automatically or manually, periodically corrected corresponding to the wear on the tool. Images of the workpiece 106 and template 101 may be episcopically projected on to a screen 87 by means of a lamp 94 disposed in a tube 93 in which is a prism structure 96 comprising a part transparent surface 97 which reflects some of the light on to the upper surface of the workpiece and transmits the remainder through a reflecting surface 98 on to the template 101. The light falling on the template and workpiece is reflected back into an objective 90 which forms images of the workpiece and template on the screen 87. The workpiece may also be diascopically projected by a lamp 107 disposed beneath the workpiece and a similar lamp used to diascopically project the template. Differently coloured workpiece and template images may be obtained by using differently coloured lamps or on interference filter. An objective may be interposed in the reflection path of the template to correct for different path lengths. The tool, as shown a grinding wheel on a slide 104, is reciprocated parallel to the optical axis 109 of the objective 90, pivoted about an axis parallel to the optical axis, and moved perpendicular to the axis on slides 81. The tube 93 to which the template is connected is carried by the grinding head 82 so that the template follows all movements of the grinding wheel other than that parallel to the optical axis; the image of the template thus always remains in focus on the screen. The template is adjustably mounted on a member 100 which is movable along an inclined path between upper and lower abutments, being moved periodically during a grinding operation from its upper operative position to abut the abutment 110 where the grinding wheel cuts a contour of its shape on the template such that, when the member is returned to the upper abutment, the template is in focus on the screen in exactly the same position as when it occupied the lower position and thus represents the position and current shape of the grinding wheel. Compensation for wear of the grinding wheel is provided by adjusting the template forwardly on the member 100 prior to moving the latter to contact the abutment 110. If desired, the workpiece itself may be used instead of the abutment 110. The movements of the member 100 may be controlled manually or effected automatically at predetermined intervals by means of cams or gears driven from the reciprocating movements of the grinding wheel. In the arrangement shown in Fig. 7, images of the template 9 and workpiece 4 may be projected either diascopically or episcopically, or by a combination of both, from a single lamp by inserting or removing shutters 260, 290 in the light beams 26, 29 and by using or not using, according to circumstances, a reflector 224 or a part-transparent reflector 221 beneath the workpiece 4. The part-transparent reflector 221 may act as an interference filter to pass differently coloured light to the workpiece and template. In a further modification, the template is disposed to engage the grinding wheel diametrically opposite to the workpiece and moved on a horizontal slide, a roof prism correcting for lateral inversion; an adaptation of this arrangement for undercut grinding or for grinding a conical profile, the base of which is of complicated form i.e. drawing dies or nozzles, is shown in Fig. 6, wherein the axis of reciprocation of the grinding wheel is inclined to the optical axis and the template. which may be transparent and coated on its under surface, is disposed diametrically opposite the workpiece and arranged to be pivoted through 180 degrees from one position, where it is corrected for the wear on the grinding wheel, to a second position in the light beam. For grinding cylindrical workpieces in which the grinding wheel is not reciprocated and therefore remains in the image plane, a portion of the surface of the grinding wheel diametrically opposite the workpiece 4, Fig. 5, may serve as the template and be diascopically projected on to the screen 2 at the same time as the latter receives an image of the workpiece; in this case, the prisms previously employed are replaced by mirrors and the beam laterally displaced from the optical axis. A simpler form of the invention comprises a ray splitter and reflector rotatable about the optical axis of the objective and coupled with the swinging of the grinding head by a four-bar linkage.