The significance of polarised light centres in the fact that it affords a wider insight into the structure of crystals, minerals, and a number of other substances, and which could not otherwise be obtained without its aid. Its usefulness is multifold, as even glass itself, when not properly annealed, exhibits points of fracture, by a display of Newton’s rings. The knowledge thus acquired is turned to account by glass manufacturers. _Double refraction._--When an incident ray of light is refracted into a crystal of any other than the cubic system, or into compressed or _unannealed glass_, it gives rise to two refracted rays which take different paths; this phenomenon is termed _double refraction_. Attention was called to this in 1670, by Bartolin, who first observed it in Iceland spar; and the laws for this substance were accurately determined by Huyghens. Iceland spar or calc spar is a form of crystallized carbonate of lime. It is composed of fifty-six parts of lime and forty-four parts of carbonic acid, and is usually found in rhombohedral forms of crystallization. To observe the phenomenon of double refraction, a rhomb of Iceland spar may be laid on a page of a printed book, when all the letters seen through it will appear double; the depth of the blackness of the letters is seen to be considerably less than that of the originals, except where the two images overlap. In order to state the laws of the phenomena with precision, it is necessary to attend to the crystalline form of Iceland spar, which has equal obtuse angles. If a line be drawn through one of these corners, making equal angles with the three edges which meet there, it, or any line parallel to it, is called the _axis_ of the crystal; the axis being, properly speaking, not a definite _line_ but a definite _direction_. The angles of the crystals are the same in all specimens. If the crystal is of such proportions that these three edges spoken of are equal, as in the smaller crystal (Fig. 176), the axis is the direction of one of its diagonals, as represented. Any plane containing (or parallel to) the axis is called the _principal plane_ of the crystal. In the next diagram, Fig. 177, the line appears double, as _a b_ and _c d_, or the dot, as _e_ and _f_. Or allow a ray of light, _g h_, to fall thus on the crystal, it will in its passage through be separated into two rays, _h f_, _h e_; and on coming to the opposite surface of the crystal, will pass out at _e f_ in the direction of _i k_, parallel to _g h_. The plane _l m n o_ is designated the principal section of the crystal, and the line drawn from the solid angle _l_ to the angle _o_ is where the axis of the crystal will be found; this is its optic axis. Now when a ray of light passes along this axis, it is undivided, and there is only one image; but in all other directions there are two images. [Illustration: Fig. 176.--Axis of Crystals of Iceland Spar.]