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 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.
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.
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.
POLARIZATION BY DOUBLE REFRACTION.
The name of Double-refracting or Iceland Spar is given to a very clear, limpid, and perfectly transparent mineral, composed of carbonate of lime, and found on the eastern coast of Iceland. Its crystallographic features are well described by the Rev. Walter Mitchell in his learned work on mineralogy and crystallography, and it is sufficient for the object of this article to state that it crystallizes in rhombs, and modifications of the rhomboidal system. It must not be confounded with rock or mountain crystal, which, under the name of quartz, crystallizes in six-sided prisms with six-sided pyramidal tops; quartz being composed of silica, or silicic acid and calcareous spar of carbonate of lime. Very large specimens of the latter mineral are rare and valuable, and the lion of specimens of calcareous, or double-refracting spar, is now in the possession of Professor Tennant, the eminent mineralogist of the Strand. It is nine inches high, seven and three-quarters inches broad, and five and a half inches thick; its estimated value being 100l. This beautiful specimen has been photographed, and its stereograph illustrates in a very striking manner the double refracting properties of the spar.
If a printed slip of paper is placed behind a rhomb of Iceland spar, two images of the former are apparent, and the stereograph already alluded to shows this fact very perfectly, at the same time illustrates the value of the stereoscope. Out of the stereoscope the words "Stereoscopic Magazine" appear doubled, but seem to lie in the same plane; but directly the picture is placed in the instrument, then it is clearly seen that one image is evidently in a very different plane from the other. The double-refracting power of this mineral is illustrated by holding a small rhomb of Iceland spar, placed in a proper brass tube before the orifice as at Fig. 327, from which the rays of common light are passing; if an opaque screen of brass perforated with a small hole is introduced behind the rhomb, then, instead of one circle of light being apparent on the screen, two are produced, and both the rays issuing in this manner are polarized, one being termed the ordinary and the other the extraordinary ray. (Fig. 327.)
Fig. 327.