Having discussed the various modes of obtaining polarized light, the next step is to arrange an apparatus by which certain double refracting crystals, and other bodies, shall divide a ray of polarized light, and then by subsequent treatment with another polarizing surface, the divided rays are caused to _interfere_ with each other, and afford the phenomena of colour. Bodies that refract light singly, such as gases, vapours or liquids, annealed glass, jelly, gums, resins, crystallized bodies of the tessular system, such as the cube and octohedron, do not afford any of the results which will be explained presently, except by the influence of pressure, as in unannealed glass, or a bent cold glass bar. By compression or dilatation, they are changed to double refractors of light. The bodies that possess the property of double refraction (though not to the visible extent of Iceland spar), are all other bodies such as crystallized chemicals, salts, crystallized minerals, animal and vegetable substances possessing a uniform structure, such as horn and quill; all these substances divide the ray of polarized light into two parts, and by placing a thin film of a crystal of selenite (which is one of the best minerals that can be used for the purpose) in the path of the beam of polarized light, coming either from the glass plates, as in No. 2, (Fig. 325), page 338, or from a slice of tourmaline, and then receiving it through the ordinary focusing lenses or object-glasses of the oxy-hydrogen microscope, no colour is yet apparent in the image of the selenite on the screen, until [Page 344] another tourmaline, or a bundle of glass plates, is placed at an angle of 56° 45´, and at right angles to the plane of reflection of the first set of plates; then the most gorgeous colours suddenly appear over all parts of the film of selenite as depicted on the screen, like other objects shown by the oxy-hydrogen microscope. (Fig. 333.) [Illustration: Fig. 333. Duboscq's polarizing apparatus, A. The light and the condenser lens. B. The plates of glass at the proper angle, C. The selenite object, D. The focusing lens. E. The second bundle of plates of glass called the analyser, F. A stop for extraneous rays of light, G. The image of the film of selenite most beautifully coloured.