a. The condensers. b. The hole in the brass screen or stop. c. The rhomb of Iceland spar. o. The ordinary, and e the extraordinary, ray, both of which are polarized light.
The polarizing property of the rhomb is perhaps better shown by the next diagram, where a b represents the obtuse angles of the Iceland spar, and a line drawn from a to b, would be the axis of the crystal. The incidental ray of common light is shown at c, and the oppositely polarized transmitted rays called the ordinary ray o, and extraordinary ray e, emerge from the opposite face of the rhomboid. If a black line is ruled on a sheet of paper as at k k, and examined by the eye at c, it appears double as at k k and j j. (Fig. 328.)
Fig. 328.
Rhomb of Iceland spar.
The cardboard model is again useful in demonstrating the polarization of light by double refraction, and if a model of a rhomb of Iceland spar is made of glass plates, one face of which has an aperture like a cross, and the other a horizontal and perpendicular slit, as at Nos. 1 and 2 (Fig. 329), the production of the ordinary and extraordinary rays is demonstrated in a familiar manner, and is easily comprehended.
Fig. 329.
No. 1. One face of the model rhomb to admit the transversal vibration, represented by the cardboard model.—No. 2. The opposite face of the rhomb, from which issue the polarized, ordinary, and extraordinary rays.—No. 3. Side view of the model.
In Newton's "Optics" we find the following description of Iceland spar:—"This crystal is a pellucid fissile stone, clear as water or crystal of the rock (quartz), and without colour.... Being rubbed on cloth it attracts pieces of straw and other light things like amber or glass, and with aquafortis it makes an ebullition.... If a piece of this crystalline stone be laid upon a book, every letter of the book seen through it will appear double by means of a double refraction."