690,695. Surveying instruments. FILOTECNICA SALMOIRAGHI SOC. PER AZIONI. Feb. 16, 1950 [Feb. 25, 1949], No. 3987/50. Class 97 (ii). The invention relates to a direct-reading tacheometer of the kind having a distance measuring telescope of the constant length type rotatable in a vertical plane about a horizontal axis and provided with an objective system having a focal length f# and a vertical circular tangential scale of mean radius r coaxial with the said axis of rotation and which is projected into the telescope field by an optical system having a magnification I, the intervals between any two succeeding marks of the scale (which marks, with respect to the zero mark of the scale in the horizontal plane passing through the said axis of rotation, subtend at the centre of the scale angles α 1 , α 2 ) being such that tan α 1 -tan α 2 is constant and equal to f#/200Ir=1/200 According to the invention a closely approximately accurate instrument is provided of reduced dimensions by reducing the factors f #, r and I in an instrument in which f#/200Ir = N/200 N having a value greater than unity or equal to or less than 5. The usual pair of horizontal hairs of the reticle between which the length intercepted on a stadia rod when viewed through the telescope, is read, are omitted and two lines of the tangential scale projected on to the image plane of the telescope are utilized in their stead. The tangential scale may also be given an altimetric function in which case the value of N is an integral number 2-5 and scale is provided with numbers which represent the per cent slope in respect to the horizontal plane passing through the zero mark. When N has a value other than an integral number a separate scale with an altimetric function must be provided. As shown in Fig. 3 an alidade 1 rotatable in a horizontal plane about a vertical axis 2 carries a fixed length telescope 3 rotatable about a horizontal axis 4-4<SP>1</SP>. The telescope is provided with an objective system 5 and a movable focusing lens 6 controlled by means of a knurled sleeve 7, with an eye-piece 9 and with a micrometer 8. The micrometer 8 comprises a glass 81 having a cross-reticle engraved thereon and two rectangular prisms 8<SP>11</SP> glued together at their hypothenuse faces, one of the faces being half-silver plated, or silver-plated or aluminium plated in zones side by side or overlapping the reticle so that when viewed through the eye-piece these zones appear as bright backgrounds. The tangential scale is marked on the cylindrical surface of a transparent cylindrical armilla 10 as lines parallel to the cylinder axis, and the armilla 10 is fixed to a metal disc 11 coaxial with the axis 4-4<SP>1</SP> mounted on a pin 411 adjustably fixed to the alidade 1 so that the zero of the scale may be brought into the horizontal plane passing through the axis 4-4<SP>1</SP>. An auxiliary objective lens system 16 and deflecting prism 17 projects the image of the scale on the armilla 10 into the field of view of the telescope. To this end light reflected from an adjustable mirror 20 is deflected by a prism 19 located at the junction of longitudinal and transverse bores in the pin 4<SP>11</SP> through the scale portion of the armilla to the objective 16. Fig. 5 shows the field of view of the telescope, in the case when N is not an integral number, with the image of the stadia; in zone a the image of the telemetric function tangential scale; in zone b the image of an altimetric scale and in zone c the numbers indicating the per cent slopes of the corresponding lines in zone b. The vertical line 21 and horizontal line 22 comprise the image of the cross reticle. The upper and lower marks in zone a intercept on the stadia a length equal to <SP>1</SP>/ 100 th of the horizontal distance D from the centre 2 of the telescope to the stadia rod. To obtain the height h the line 22 is made to coincide with one of the marks in zone b. The number opposite this line in zone c gives the per cent slope, which when multiplied by D/100 gives the height h and the height reading on the stadia where the line 22 crosses it subtracted from h gives the height H of the foot of the stadia above the horizontal plane through the axis 4-4<SP>1</SP> of the telescope. When N is an integral number the images in zones a and b are similar, since a single tangential scale only is used. In an alternative construction the tangent scale is carried as radial marks of mean radius r on an annular glass plate concentric with the axis 4-4<SP>1</SP>, the surface of the plate carrying the marks lying in the image plane of the objective system of the telescope, in which case the value for I in the above formula is unity, and a single tangential scale is used, utilizing for the value of N an integral number. The invention is said also to be applicable generally to topographic instruments with constant length telescopes such as diopters and slope levels.