Microscopes other than optical microscopes; diffraction apparatus

90.12 - Microscopes other than optical microscopes; diffraction apparatus. - Microscopes other than optical microscopes; diffraction apparatus 9012.90 - Parts and accessories 9012.10 This heading includes : (A) Electron microscopes differ fiom optical microscopes in that they use a beam of electrons instead of light rays. The normal type of electron microscope is an assembly of the following devices usually enclosed in a cornmon frame as a unit : (I) A device (known as an electron gun) for emitting and accelerating the electrons. (2) A system (playing the art of the optical system of of electrostatic or e ectromagnetic " lenses " coils carrying a current); these act as usually also a further so-called field " which serves to vary the range of scanned field. P (3) The specimen stage. (4) The vacuum urnp unit which maintains a vacuum in the electron tube; these are sometimes sel!-contained units connected to the appliance. (5) The elements for visual observation on a fluorescent screen and for photographic recording of the image. (6) Control stands and panels bearing the elements controlling and regulating the electron beam. This headin also includes scanning electron microsco es in which a very fine beam of electrons is 'rected repeated1 onto different points of e sample. Information is obtained by measuring, for example, e electrons transmitted, the secondary electrons emitted, or the optical rays. The result may then be displayed on a monitor screen which can be incorporated in the microscope. tK tE The electron microscope has many uses both in the field of ure science (biological or medical research, composition of matter, etc.), and in industria technique (examination of h e s , dust, textile fibres, colloids, etc.; examination of the structure of metals, paper, etc.). f (B) Proton microscopes. In lace of electrons, these employ rotons which have a wavelength 40 times shorter than trie former. A correspondingly ligher separating power is thus obtained and this permits the production of even more highly magnified images. The structure and functioning of the proton microscope do not differ appreciably from those of the electron microscope; the electron gun is replaced by a proton gun and the source used is hydrogen. (C) Electron diffraction apparatus. B means of a beam of electrons directed at a specimen, these produce diffraction patterns w&ch are photographed. The dimensions, orientation and atomic arrangement of the crystals of the specimen examined can be calculated fiom the diameter, intensity and sharpness of the rings in the pattern. This apparatus, which is chiefly used for studies on corrosion, lubrication, catalysis, etc., does not differ ap reciably in principle fiom an electron microscope, and has the same coils, specimen holder, essential elements felectron gun, cathode-ray tube, electro-rnagnet~c etc.). Moreover it should be noted that certain electron microscopes may be equipped with a diffraction chamber and can therefore perform a double function (visual exarmnation and production of a diffraction pattern). PARTS AND ACCESSORIES Subject to the provisions of Notes 1 and 2 to this Chapter (see the General Ex lanatory Note), parts and accessories suitable for use solely or princj ally with microscopes, o er than optical microscopes, or diffraction apparatus are also class1 led here; exam les are the Erame and its constituent chambers and the specimen stage. On the other hand, the eadin excludes vacuum pumps beading 84.14), electrical equipment (batteries, rectifiers, etc.) &ha ter 851, and electrica measuring instnunents (voltmeters, milliammekrs, etc.) (beading 90.30fl F R & -