168,316. Wild, H. Aug. 28, 1920, [Convention date]. Theodolites.-Relates to instruments, such as are described in Specification 118/08, provided with optical means for bringing into juxtaposition in the field of view of a reading microscope the images of diametrically opposite points of a graduated circle. According to the invention the arrangement is such that the graduations of the two portions of the circle increase in opposite directions in the field of view, and the optical parts include a deviating device such as a system of rotatable plane-parallel glass plates, by means of which the images can be displaced relatively to each other, preferably by equal and opposite amounts, until the graduation marks in the two contiguous images form continuous lines. The displacement of the deviating device then affords a measure of the smaller units of the angle to be read. The Figures show a theodolite in which the images of the horizontal and vertical scales and also that of a scale for the deviating device are seen together in the field of view as shown in Fig. 6. The horizontal scale is marked on the outer surface of a glass ring d in the base of the instrument and is illuminated from outside by means of reflecting prisms e<1>, e<3>. Light from the scale is reflected by prisms e and passes through a lens f, forming the objective of the microscope, along the axis of the hollow pivot c<1> of the instrument to a reflecting-prism f<1>, the two raypencils being slightly inclined to each other as shown. The vertical scale is similarly arranged on a glass ring h and the images of diametrically opposite points are projected by prisms i through an objective k to a prism k<1> which reflects the slightly divergent ray pencils along a hollow part of the mounting c. The four ray pencils are received by a prism l which renders them approximately parallel to each other. The rays then pass through the deviating device comprising four plano-parallel glass plates m<1> - - m<4> arranged in pairs in stirrups n<3>, n<4> rotatable around vertical axes n<1>, n<2>. The stirrups can be turned through equal angles in oposite directions by rotating a pinion p which gears with two equal pinions q fast with sleeves q<1> acting as nuts to longitudinally displaceable threaded pins o the ends of which bear on lugs n<5>, n<6> carried by the stirrups. The pinion p carries a cylindrical scale p<4> which is illuminated from outside by a prism p<5>. The rays from the instrument scales are reflected by a prism r to a lens s which also receives the rays from the scale p<4>, the light being then received by a reflecting prism s<1> which can be rotated by an arm s<2> so that the images can be seen in either of the eyepieces s<3>. The prism r carries wedges r<1>, r<2>, r<3> to compensate for the initial divergence of the rays of each of the two systems. Fig. 6 shows the manner in which the images are presented, the image of the vertical scale being on the left, that of the horizontal scale in the centre and that of the scale p<4> on the right. Indices are provided at the points on the scales where the readings differ by 180‹ and also an index for the scale p<4>. The figure shows the manner of setting the deviating device for obtaining a reading of the horizontal scale, the head p<1> being turned until the graduation lines of the two half images are continuous. The desired reading is then 10‹ 53' 47".