Patent GB-586392-A (GB)
Title: Improvements in or relating to optical angle blocks of the prism type
IPC: G02B27/00
Filed: 1944-06-02
Abstract: 586,392. Optical prism blocks. TAYLOR, TAYLOR, & HOBSON, Ltd., and WARMISHAM, A. June 2, 1944, No. 10667. [Classes 97 (i) and 97 (iii)] An optical angle block of the prism type has one or more internally reflecting but non- transmitting surfaces, an external surface which is rendered both internally and externally reflecting or semi-reflecting, and an external surface which transmits but is rendered substantially non-reflecting. It may consist of a single prism with relatively adjustable elements (e.g. thin wedges) to compensate for any slight manufacturing error of prism angle ; or of more than one prism, in which case error in the prism-block angle is corrected by relative adjustment of the individual prisms. The block may be used with apparatus for subdividing a circle into equal angular subdivisions. As shown, the block comprises a right-angled isosceles prism having its hypotenuse face A3 internally reflecting but non- transmitting, its external face A1 metallised to render it wholly reflecting both internally and externally, and its external face A2 transmitting but rendered substantially non-reflecting as by applying a reflection-reducing skin or film. With face A2 are associated a pair of wedges B, C capable of opposite relative angular adjustment to correct, error of prism angle, and permanently secured relatively to the block in their adjusted position, the faces B1, B2, C1, C2 of the wedges being transmitting but rendered non- reflecting. If desired, one wedge may be associated with each of the faces A1, A2. The block is shown mounted on the rotatable head D of the aforementioned subdividing apparatus, with its reflecting faces parallel to the axis of rotation of the head, and with a fixed autocollimating telescope E directed towards said axis and intersecting it at right angles. With the head locked against turning, the block -is turned about said axis until an image of the telescope is seen by external reflection in face A1. This gives the zero position D1 from which the subdivision is to begin. The block is now locked to the head, and the head is unlocked and turned clockwise with the block until the image of the telescope is seen through the prism by internal reflection at the face A1. This setting corresponds to a 90 degree turn of the head, and the appropriate marking may be made in the machine with the aid of a fiducial mark F1 after locking the head. Similar operations are carried out for effecting a further 90 degree turn of the head, and so on until the whole circle has been subdivided. Modified blocks are described consisting of a single four-sided prism with two internally reflecting faces to provide a 60 degree turn, a single four-sided prism with one such reflecting surface to provide a 120 degree turn, and a pair of isosceles triangular prisms each with one internally-reflecting surface, the inner transmitting faces of the prisms (which are rendered non-reflecting) being set at a small angle to each other to adjust the deviation angle of the whole block accurately to the desired value. The face of the block which is externally reflecting may be partially reflecting ; in the case of a 90 degree block, for example, this permits the block to be used as an optical square.