[Illustration: Fig. 325.--No. 1. A is the lime light. B. The condenser lenses. C. The beam of _common_ light. Here the glass plates are removed.--No. 2. A. Lime light. B. The condenser lenses. C C. The bundle of plates of glass at an angle of 56° 45´. D is the ray of light polarized by reflection from the glass plates, C C, and E is the beam of polarized light by single refraction, having passed through the bundle of plates of glass, C C.] In the year 1810, the celebrated French philosopher, Mons. Malus, while looking through a prism of Iceland spar, at the light of the setting sun, reflected from the windows of the Luxemburg palace in Paris, discovered that a beam of light reflected from a plate of glass at an angle of 56 degrees, presented precisely the same properties as one of the rays formed by a rhomb of Iceland spar, and that it was in fact polarized. _One_ of the transversal waves of polarized light of the common light, being reflected or thrown off from the surface of the glass, whilst the other and second transversal vibration passed _through_ the plate of glass, and was likewise polarized in another plane, but by _single refraction_, so that the experiment illustrates two of the modes of polarizing light-—viz., by reflection, and by single refraction. This important elementary truth is beautifully illustrated by Mr. J. T. Goddard's new form of the oxy-hydrogen polariscope, by which a beam of common light traverses a long square tin box without change; but directly a bundle of plates of glass composed of ten plates of thin flattened crown glass, or sixteen plates of thin parallel glass plates used for microscopes, are slid into the box at an angle of 56° 45´, then the beam of common [Page 339] light is split into two beams of polarized light, which pursue their respective paths, one passing by single refraction through the glass, and the other being reflected, and rendered apparent by opening an aperture over the glass plates, and then again by using a little smoke from brown paper, the course of the rays becomes more apparent. [Illustration: Fig. 326. A A. Model in wood of a bundle of plates of glass at an angle of 56° 45´. B. Beam of common light, with transversal vibration. C. Light polarized by reflection. D. Light polarized by refraction.] The same truth is well illustrated by the cardboard model wave and a wooden plane with horizontal and perpendicular slits, placed at an angle of 56° 45´, as at Fig. 326. POLARIZATION BY DOUBLE REFRACTION.