The Microscope. Its History, Construction, and Application 15th ed. / Being a familiar introduction to the use of the instrument, and the study of microscopical science

by Jabez Hogg

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.

Fig. 176.—Axis of Crystals of Iceland Spar.

Fig. 177.—A Rhomb showing the passage of Rays of Light.