GB856621A - Improvements in or relating to polarising microscopes - Google Patents

Abstract

856,621. Polarizing microscope. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 17, 1957 [July 20, 1956], No. 22611/56. Class 97(1) A polarizing microscope includes between a polarizer and rotatable analyser a corrector comprising one or more optical elements producing barrel distortion of the state of polarization of the beam passing through the microscope substantially to balance the pincushion distortion of such state caused by the other optical elements so that parallel plane polarized light reaches the analyser in the absence of a specimen to be observed. A beam of light 1 passes through a diaphragm 11 and thence through a polarizer 2 with a state of polarization indicated by 8 (i.e. plane polarized) to the corrector which includes a slab of birefringent uniaxial material mounted with its optic axis parallel to the axis of the optical system and producing a state of polarization as shown at 13; the corrector further comprises a quarter wave plate 14 which modifies the polarization pattern to that shown at 15 which is barrel distortion sufficient to nullify the pincushion distortion produced by the microscope lens system so that - for a certain axial position of the rotatable analyser 7 - complete extinction is obtained throughout the wavefront. In a second embodiment, the corrector consists of a single half wave plate of biaxial birefringent material with the refractive index parallel to the optical axis of the optical system differing considerably from those on the plane normal to this which are of approximately equal magnitude. Fig. 4 shows a further form of corrector consisting of a plate of optically active material of non uniform thickness - the surfaces being such as to distort the polarization in the required barrel manner. The upper face 20 is convexcylindrical and the lower face 21 of concavecylindrical form - the radii of the two cylinders being equal and their axes at right angles and disposed at 45 degrees to the plane of polarization of the light issuing from the polarizor indicated by the line 22. A further form of corrector comprises a piece of uniaxial birefringent material of the shape shown on Fig. 4, with optic axis perpendicular to the microscope axis and at 45 degrees to the plane of polarization of the rays issuing from the polarizor, in conjunction with which is a plane parallel plate of uniaxial birefringent material together with a quarter wave plate. For use with a strongly converging or diverging incident beam, the corrector may consist only of a half wave plate of uniaxial birefringent material with the slow direction either parallel or perpendicular to the plane of polarization of the incident ray. Also in the case of a strongly converging or diverging beam, the corrector may comprise one or more lenses together with a half wave plate with either its slow or fast direct parallel to the plane polarization.