GB685286A - Improvements in and relating to microscopy - Google Patents

Abstract

685,286. Apparatus for producing images of small objects by photographic means. BRITISH THOMSON-HOUSTON CO., Ltd. Jan. 19, 1949 [Dec. 17, 1947], No. 33313/47. Classes 97 (i), 98 (i) and 98 (ii). [Also in Group XL (c)] Apparatus for producing by means of a diffraction photograph a magnified image of an object comprises a means for producing photographically with rays of a very short wavelength, e.g. an electron beam (considered from its wave aspect), a diffraction diagram of the object which is to be magnified, the diffraction being carried out in such a manner that the diagram is formed substantially by interference of diffracted wavelets with primary undiffracted waves from the same source, a means for recording this diagram, and a means for producing from the record with rays of a wavelength longer than that of the waves for producing the diffraction diagram an image of the object which is magnified at a ratio equal to the ratio of the two wavelengths. In the apparatus for producing the diffraction diagram, Fig. 3, the radiation preferably consists of a monochromatic beam of electrons falling on an aperture 3. An image of this aperture, or of the cross-over point in an electron gun, is formed by magnetic lenses 4, 5, 6 just behind, or just in front of, the object O to be examined, the size of the image being equal to or smaller than the resolving limit of a microscope having an illuminating beam of the same aperture angle. Every point in the object O produces a diffraction pattern at a photographic plate 1, which is sufficiently greater than the shadow image of the object to record diffraction patterns up to the limit of resolution of the plate. The exposed plate is processed and may be used as a negative, but preferably a diapositive is produced. In the apparatus for producing the image, Fig. 4, monochromatic visible or ultra-violet light illuminates an aperture 10, an image of which is formed by lenses 11, 12. The size of the image bears the same ratio to the size of the electron image as does the wavelength of the light to the de Broglie wavelength of the electrons. Aberrations of the electron image produced by the magnetic lenses should be reproduced in the image formed by the lenses 12, 13 or equivalent alterations in the wavefront effected by means, preferably adjustable, introduced in front of the image. If a plate bearing the diffraction pattern enlarged in the ratio of the wavelengths were introduced into the light beam, the distance from the aperture to the plate being increased in the same ratio, the wavefront of light passing through the plate would be substantially the same as that which would result from an object representing an enlargement of the original object in all its dimensions in the ratio of wavelengths placed at a distance from the aperture image greater than the original image in the same ratio. To avoid the use of a large plate at a great distance, a lens 13 is introduced together with plate 1 or a corresponding diapositive, such that an image of the plate is produced at the required distance and of the required size. Rays passing through the plate are focused by a lens 14 to form an enlarged image of the object which may be viewed or photographed. The real image of the virtual source (i.e. the aperture image) may be covered by a small black disc to provide dark field illumination, or by a quarterwave plate to provide phase contrast.