Patent GB-715752-A (GB)
Title: Improvements in phase-contrast microscopy
IPC: G02B21/14
Filed: 1951-04-23
Abstract: 715,752. Phase contrast microscopes. SOC. ANON. DE VENTE DES INSTRUMENTS DE GEODESIE H. WILD. April 23, 1951 [April 26, 1950, April 3, 1951], No. 9439/51. Class 97(1) A phase contrast microscope comprises a diaphragm disposed in the front focal plane of the sub-stage condenser and a phase plate disposed in the plane in which the microscope objective forms a real image of the diaphragm aperture and is characterized in that first colour filter is disposed in the light path to a specimen to be examined and that a second colour filter is provided in association with the conjugate or complementary area of the phase plate, these filters transmitting a common band of wavelengths which is in amplitude equal to or less than that of the wavelength range transmitted by the first filter and the mean wavelength of this common band being different from that of the band transmitted by the first filter. As air alternative to the first filter an optical device may be used this device including means for splitting the light beam from the source into two split beams. a colour filter in the path of each split beam and means for re-combining the split beams. As shown in Fig. 4, light from a source 14 is collimated at 15 and passes through a filter 27 and the annular slit 11 of a diaphragm 12 disposed in the front focal plane of a condenser 13. A specimen to be examined is placed at 16 and the diaphragm is imaged in the back focal plane of the objective 17 where a phase plate 18 is disposed. The conjugate area of this plate may be constituted by a thin annulus of a transparent substance such as gelatin coloured to form the second colour filter, the annulus being surrounded by a resin and the whole being sandwiched between glass plates. Alternatively the conjugate area of the phase plate may be an interference filter such as an annulus of cryolite sandwiched between semi-transparent aluminous layers and supported on a glass plate or on the fan of a lens. The first colour filter 27 is preferably an interference filter comprising a wedge-shaped transparent layer sandwiched between semi-transparent layers so that the wavelength of the rays transmitted thereby can be varied by moving the filter in a direction perpendicular to the optical axis. This filter may alternatively be of uniform thickness and be mounted for rotation about an axis perpendicular to the optical axis. An optical device for use instead of the first filter 27 comprises, as shown in Fig. 11, a beam splitting prism 36, colour filters 38, 39 in the path of each split beam and a prism 42 for re-combining the split beams. A third colour filter may be associated with the complementary area of the phase plate to transmit a wavelength band different from that transmitted by the second filter associated with the conjugate area of the phase plate. In a further form, the microscope includes a phase plate which works by reflection and comprises a glass plate 59, Fig. 13, having an opaque reflecting layer 64 over all but the conjugate area of the phase plate, this area being in the form of an interference filter having a cryolite layer 61 sandwiched between semi-transparent metallic layers 62, 63. Such a plate may be formed on one surface of a prism which totally reflects the light reflected from the phase plate, this surface of the prism being inclined or perpendicular to the optical axis of the microscope. Specifications 15719/48 (as open to public inspection under Sect. 91) and 647,191 are referred to.