Patent GB-899922-A (GB)
Title: Improvements in or relating to optical instruments with adjustable magnification
IPC: G02B21/22
Filed: 1959-08-20
Abstract: 899,922. Vari-focal optical systems; stereoscopic microscopes. BECK Ltd., R. & J., SETTERINGTON, R., and GRANT, R. E. Nov. 15, 1960 [Aug. 20, 1959], No. 28555/59. Class 97(1). An optical instrument (for example a stereoscopic microscope) comprises two lenses or lens groups between which there is disposed an afocal system and means are provided for rotating the afocal system about an axis at right angles to the optical axis between two angular limiting positions in both of which the afocal system is in optical alignment with said two lenses or lens groups. Means are also provided for adjusting the axial distance apart from the lenses in the afocal system so that axial adjustment from one axial limiting position to another axial limiting position varies the magnification of the afocal system in one sense, and adjustment back from said other axial limiting position to the first axial limiting position, after rotation of the afocal system, results in a continued change of magnification in the same sense. As shown in Fig. 4, the optical instrument includes two lenses 10 and 11 and a prism 13 between the lens 11 and the focal plane 12 of an eyepiece. Between lenses 10 and 11 is an afocal system made up of positive lenses 14 and 16 and a central negative lens 15. Of this afocal system the lenses 14 and 15 are relatively movable axially so as to vary the magnification. And the afocal system as a whole is rotatable about an axis 17 perpendicular to the optical axis. In use the lenses 14 and 15 are moved through their position of minimum magnification to a limiting position in which the magnification is (say) xl. The afocal system is then rotated through 180 degrees and axial movement of the lenses 14, 15 is continued so as to increase the magnification from xl to x4. The system shown in Fig. 4 may be duplicated so as to constitute a stereoscopic microscope, the lens 10 being large enough to act for both systems. Such a microscope is shown in Fig. 1 wherein the housing 19 accommodates the lens 10 and contains spaced discs 30 having radial slots 31 and 32 in which tongues on mountings 33, 34 for the lenses 14 and 15 respectively project. Pins 43, 44 secured to these tongues ride in cam slots 27, 28, Fig. 3, in discs 24, which are secured to rotatable knobs 22, journalled in the housing 19. The lenses 16 are fixably carried by the two discs 30, one of which has a slot 49 engaged by a pin 48 projecting from one cam disc 24. In use, starting from the position of minimum magnification with the pin 48 at one end of the slot 49, the knobs 22 are turned clockwise so as to increase the magnification. This turning movement is continued until the pin comes into contact with the other end of the slot 49 as shown in Fig. 3. In order to increase the magnification still further, further clockwise rotation of the knobs 22 is effected so that the disc 30 is rotated through 180 degrees to reverse the position of the lens in the afocal system. Thereafter to increase the magnification the knobs 22 are rotated anti-clockwise until the pin 48 again reaches its original position in the slot 49. The two discs 24 are coupled together by gearing and the angular limiting positions of the two discs are determined by spring detent arrangements. The housing 19 is adjustable for focusing by the usual rack and pinion mechanism.