1,223,101 Surveying apparatus. BARR & STROUD Ltd. Jan.24, 1966 [Feb.13, 1965], No.6328/65. Heading G1F. A sight-taking periscope includes a head in which are mounted first and second elevating prisms, means for independently rotating these prisms to view upwardly from the horizontal, an angular scale of altitude rotatable with one prism, and a scale reading optical system having a head rotatable with the other prism, whereby a measure of the angle between the lines of sight of the prisms is provided. In the embodiment described, one prism is of sufficient size to provide the maximum useful aperture (say 80mm) which the observer can accept taking into consideration the magnification of the viewing system (say X 20) and has sufficient light gathering power to enable first magnitude stars to be seen well into daylight and the horizon to be seen well into darkness. The second prism, due to space limitations in the head of the periscope, is considerably smaller and provides a pupil of, say 25mm, this however being sufficient to enable stars to be seen at night and the horizon in daytime. The role of the two prisms, in respect of the object viewed, is thus changed as between daylight and darkness. The angular scale and the scale reading optical system are such as to enable readings to be taken across a diameter of the scale, the images of both sides of the scale being combined in the head of the scale reading optical system. This system includes a dove prism rotatable upon rotation of the corresponding prism and serving to stabilize the combined scale image against rotation to facilitate reading. In use the prisms are sighted on the horizon and a star or other bright body and the scale reading is then an accurate measure of the elevation of the bright body. For purposes of dip and refraction error compensation, one prism is capable of rotation about its axis to the extent of viewing horizontally in the opposite direction to the other prism. By this means, the angle subtended at the instrument head, by diametrically opposite points on the horizon, can be measured and the amount by which this angle exceeds 180 degrees is a measure of the combined dip and refraction angle.
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