Patent GB-639014-A (GB)
Title: Improvements in or relating to microscopy
IPC: G02B21/14
Filed: 1947-08-05
Abstract: 639,014. Microscopes. SMITH, F. H. Aug. 5, 1947, Nos. 21314, 21995, 21996, 22665, 25761 and 29918. [Class 97 (i)] To facilitate examination of transparent objects the illuminating beam is divided, e.g. by polarizing or reflecting systems, into two distinguishable beams at least one of which passes through the object so that the path lengths of the beams differ due to the nature of the object, and means are provided for subsequently combining the beams so as to produce interference effects. The beams may be distinguished by their states of polarization, degree of convergence or divergence or general direction of propagation. As shown in Fig. 2, the illuminating beam is plane polarized by polarizer 15 and then passes through a Wollaston prism, the dispersion of which may be corrected by a prism 27a, or a Rochon prism, which divides the beam into two divergent beams plane polarized at right angles which are focused by the condenser 11 at two transversely displaced points 21, 22 at one of which the object is located. The beams are recombined by a further Wollaston or Rochon prism 24 and then pass to the eyepiece 12 and an analyser 25 which renders the polarization planes of the beams parallel to allow interference. A Babinet compensator 19 allows adjustment of the light paths of the beams. The dividing prism may space the focii 21 and 22 so widely that separate objectives and condensers are used for each beam, auxiliary prisms directing the beams as required. As an alternative, the mirror 13 may be semitransparent and located between the collimator 26 and prism 24. This reflects light through the object on to a concave mirror which reflects the light back along the same path into the eyepiece. The lower double image prism is omitted so that prism 24 serves both as the beam dividing and combining means. Alternatively, the prisms may be replaced by birefrigent lenses, the lower of which provides two axially separated focii and the upper acts as the beam combining means. The prisms may be rotated to vary the light path or a rotatable birefrigent plate may be provided for this purpose in addition to the compensator 19. In a modification, the lower prism may be disigned to produce two widely divergent beams which are directed through identical microscope systems, one of which includes the object, the beams being subsequently recombined into a single eyepiece. A system for producing two divergent beams plane polarized at right angles comprises a glass prism 35, Fig. 3, carrying a thin layer 34 of calcite and separated by a wedge-shaped airspace from a reflector 33. Plane polarized light from the analyser 31 is directed upon the calcite-air surface at an angle greater than the critical angle for one ray, but less than that for the other. The first ray is consequently totally reflected, but the second passes out of the surface and is reflected as shown by the reflector 33 to form a beam diverging from the first. A system using unpolarized light is shown in Fig. 6. The light after reflection at the mirror 13 passes to a glass body 40 having a conical semi-reflecting surface 42 which passes some of the light and reflects the remainder to the reflecting surface 43. The latter beam passes a lens 44 and is brought to a focus by the condenser 11 at the object plane 0, O<SP>1</SP> axially displaced from the focus of the outer annular beam. The central beam then passes a negative lens 45 and the beams are combined by a prism 41 similar to prism 40 to be viewed by eyepiece 12. A rotatable plate 46 permits adjustment of the path difference between the two beams. This system may be used with a transparent reflector between prism 41 and lens 26 to reflect light on to the object, which light, having passed the object is reflected back by a concave reflector, the beams being recombined by the prism 41, the prism 40 being omitted. Further modifications are described in which the division and recombination are effected by wedge-shaped glass plates, the beams being reflected respectively by the front and back surfaces thereof. In a further modification, a semi-transparent reflector on the optical axis reflects the beam to the object, a concave mirror reflecting this beam back through the object into the eyepiece, and the other beam passes through the reflector and is reflected back thereto by a second concave reflector. A diffraction grating may be used as the dividing and combining means and may be arranged to produce a diffracted beam or beams plane polarized at right angles to the plane of polarization of the direct beam. This grating comprises a glass plate to which a calcite plate has been cemented, ground down to halfwave thickness and then etched away in strips or circles to form a grating or zone plate, the etched portions being filled with a suitable cement.