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The two rays into which a doubly refractive stone splits up a ray of light are often differently absorbed by it, and in consequence appear on emergence differently coloured; such stones are said to be dichroic. The most striking instance is a deep-brown tourmaline, which, except in very thin sections, is quite opaque to the ordinary ray. The light transmitted by a plate cut parallel to the crystallographic axis is therefore plane-polarized; before the invention by Nicol of the prism of Iceland-spar known by his name this was the ordinary method of obtaining light of this character (cf. p. 43). Again, in the case of kunzite and cordierite the difference in colour is so marked as to be obvious to the unaided eye; but where the contrast is less pronounced we require the use of an instrument called a dichroscope, which enables the twin colours to be seen side by side.

Fig. 29.—Dichroscope (actual size).

Fig. 30.—Field of the Dichroscope.

Fig. 29 illustrates in section the construction of a dichroscope. The instrument consists essentially of a rhomb of Iceland-spar, S, of such a length as to give two contiguous images (Fig. 30) of a square hole, H, in one end of the tube containing it. In some instruments the terminal faces of the rhomb are ground at right angles to its length, but usually, as in that depicted, prisms of glass, G, are cemented on to the two ends. A cap C, with a slightly larger hole, which is circular in shape, fits on the end of the tube, and can be moved up and down it and revolved round it, as desired. The stone, R, to be tested may be directly attached to it by means of some kind of wax or cement in such a way that light which has traversed it passes into the window, H, of the instrument; the cap at the same time permits of the rotation of the stone about the axis of the main tube of the instrument. The dichroscope shown in the figure has a still more convenient arrangement: it is provided with an additional attachment, A, by means of which the stone can be turned about an axis at right angles to the length of the tube, and thus examined in different directions. At the other end of the main tube is placed a lens, L, of low power for viewing the twin images: the short tube containing it can be pushed in and out for focusing purposes. Many makers now place the rhomb close to the lens, L, and thereby require a much smaller piece of spar; material suitable for optical purposes is fast growing scarce.

Suppose that a plate of tourmaline cut parallel to its crystallographic axis is fastened to the cap and the latter rotated. We should notice, on looking through the instrument, that in the course of a complete revolution there are two positions, orientated at right angles to one another, in which the tints of the two images are identical, the positions of greatest contrast of tint being midway between. If we examine a uniaxial stone in a direction at right angles to its optic axis we obtain the colours corresponding to the ordinary and the extraordinary rays. In any direction less inclined to the axis we still have the colour for the ordinary ray, but the other colour is intermediate in tint between it and that for the extraordinary ray. The phenomenon presented by a biaxial stone is more complex. There are three principal colours which are visible in differing pairs in the three principal optical directions; in other directions the tints seen are intermediate between the principal colours. Since biaxial stones have three principal colours, they are sometimes said to be trichroic or pleochroic, but in any single direction they have two twin colours and show dichroism. No difference at all will be shown in directions in which a stone is singly refractive, and it is therefore always advisable to examine a stone in more than one direction lest the first happens to be one of single refraction. For determinative purposes it is not necessary to note the exact shades of tint of the twin colours, because they vary with the inherent colour of the stone, and are therefore not constant for the same species; we need only observe, when the stone is tested with the dichroscope, whether there is any variation of colour, and, if so, its strength. Dichroism is a result of double refraction, and cannot exist in a singly refractive stone. The converse, however, is not true and it by no means follows that, because no dichroism can be detected in a stone, it is singly refractive. A colourless stone, for instance, cannot possibly be dichroic, and many coloured, doubly refractive stones—for example, zircon—exhibit no dichroism, or so little that it is imperceptible. The character is always the better displayed, the deeper the inherent colour of the stone. The deep-green alexandrite, for instance, is far more dichroic than the lighter coloured varieties of chrysoberyl.

If the stone is attached to the cap of the instrument, the table should be turned towards it so as to assure that the light passing into the instrument has actually traversed the stone. If little light enters through the opposite coign, a drop of oil placed thereon will overcome the difficulty (cf. p. 46). It is also necessary, for reasons mentioned above, to examine the stone in directions as far as possible across the girdle also. A convenient, though not strictly accurate, method is to lay the stone with the table facet on a table and examine the light which has entered the stone and been reflected at that facet. The stone may easily be rotated on the table, and observations thus made in different directions in the stone. Care must be exercised in the case of a faceted stone not to mistake the alteration in colour due to dispersion for a dichroic effect, and the stone must be placed close to the instrument during an observation, because otherwise the twin rays traversing the instrument may have taken sensibly different directions in the stone.

Dichroism is an effective test in the case of ruby; its twin colours—purplish and yellowish red—are in marked contrast, and readily distinguish it from other red stones. Again, one of the twin colours of sapphire is distinctly more yellowish than the other; the blue spinel, of which a good many have been manufactured during recent years, is singly refractive, and, of course, shows no difference of tint in the dichroscope.

Table VI at the end of the book gives the strength of the dichroism of the gem-stones.