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 500l. 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 are very good, and it is a most curious fact that white tourmaline does not polarize. (Fig. 330.)
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.
The mineral crystallizes in long prisms, whose primitive form is the obtuse rhomboid, having the axis parallel to the axis of the prism. The term axis with reference to the earth, as shown at page 16, is an imaginary single line around which the mass rotates, but in a crystal it means a single direction, because a crystal is made up of a number of similar crystals, each of which must have its axis, thus the whitest Carrara marble reduced to fine powder, moistened with water and placed under a microscope, is found to consist chiefly of minute rhomboids, similar to calcareous spar. The smallest crystal of this mineral is divisible again and without limit into other rhombs, each of which possesses an axis. (Fig. 331.)
Fig. 331.
Fig. 331 represents a crystal, the axis of which is the direction a b. The dotted lines show the division of the large crystal into four other and smaller ones, each of which has its axis, a c, c b, d e, f g; and every line within the large crystal parallel to a b is an axis, consequently the term is employed usually in the plural number axes.
If a plate of tourmaline is held before the eye whilst looking at the sun (like the gay youth in Hogarth's picture who is being arrested whilst absorbed with the wonders of a tourmaline, which was, in the great painter's time, a popular curiosity,) it may be turned round in all directions without the slightest difference in the appearance of the light, which will be coloured by the accidental tint of the crystal, but if a second slice of tourmaline is placed behind the other, there will be found certain directions in which the light passes through both the slices, whilst in other positions the light is completely cut off.