[Illustration: 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 [Page 341] 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. [Illustration: 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." POLARIZATION BY THE TOURMALINE. This mineral was first discovered during the sixteenth century, in the island of Ceylon, afterwards in Brazil, and since that period at various localities in the four quarters of the globe. In the Grevillian collection purchased many years ago by government for the British Museum, there is a fine specimen of red tourmaline valued at 500_l._ The green tourmaline is named Brazilian emerald, and the Berlin blue tourmaline is called Brazilian sapphire; the mineral chiefly consists of sand (silica) and alumina, with a small quantity of lime, or potash, or soda, boracic acid, and sometimes oxide of iron or manganese. When light is passed through a slice of this mineral it is immediately polarized, one of the transversal vibrations being absorbed, stopped, or otherwise disposed of, the other only emerging from the tourmaline, consequently it is one of the most convenient polarizers, although the polarized light partakes of the accidental colour of the mineral. Green, blue, and yellow tourmalines are bad polarizers, but the brown and pink varieties [Page 342] are very good, and it is a most curious fact that white tourmaline does not polarize. (Fig. 330.) [Illustration: Fig. 330. Crystal of tourmaline slit (parallel to the axis) into four plates, which when ground and polished, may be used for the polarization of light.]