580,723. Range-finders. BARR & STROUD, Ltd., STRANG, J. M.,and FOSTER, C. March 14, 1941, No. 3517. [Class 97 (i)] In a range-finder comprising a tubular base casing A rotatable in a stationary bearing tube B carrying a fixed eyepiece mounting D and extending on both sides of an aperture 8 in the casing A, the casing A is mounted. on anti-friction bearings 17, 18 and annular tongues and grooves forming labyrinths F sealing the interior of the instrument from external moisture &c. are provided ; the labyrinths may be so placed as to seal against oil or grease from the bearings. As shown, the labyrinths comprise tongues formed on rings 19, Fig. 2, held against an abutment 23 on the casing A by a screw 24 ; the grooves are formed between rings 20 secured together by a bolt 21, the head of the bolt being engaged with a recess in a flange 22 on the tube B1 in such manner that the rings have slight freedom of movement. The tongues and grooves may be normal to the axis of the casing A as shown, or may be otherwise inclined or parallel thereto. A second bearing tube B2 having bearings 17, 18 may be provided. The eyepiece' mounting may house an erecting prism 9 rotated at half the speed of the casing A by a helical or scrollshaped cam 14 on the casing engaging a roller 13 on a rack bar 12 meshed with an arcuate rack on a carrier 10 for the prism.
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