In the examination of a faceted stone, of the most usual shape, the simplest method is to lay the large facet, called the table, on a glass slip and view the stone through the small parallel facet, the culet. Should the latter not exist, it may frequently happen that owing to internal reflection no light emerges through the steeply inclined facets. This difficulty is easily overcome by immersing the stone in some highly refracting oil. A glass plate held by hand over the stone with a drop of the oil between it and the plate serves the purpose, and is perhaps a more convenient method. A stone which does not possess a pair of parallel facets should be viewed through any pair which are nearly parallel.
We have stated that a plate of glass has no effect on the field. Suppose, however, it were viewed when placed between the jaws of a tightened vice and thus thrown into a state of strain, it would then show double refraction, the amount of which would depend on the strain. Natural singly refractive substances frequently show phenomena of a similar kind. Thus diamond sometimes contains a drop of liquid carbonic acid, and the strain is revealed by the coloured rings surrounding the cavity which are seen when the stone is viewed between crossed nicols. Double refraction is also common in diamond even when there is no included matter to explain it, and is caused by the state of strain into which the mineral is thrown on release from the enormous pressure under which it was formed. Other minerals which display these so-called optical anomalies, such as fluor and garnet, are not really quite singly refractive at ordinary temperatures; each crystal is composed of several double refractive individuals. But all such phenomena cannot be confused with the characters of minerals which extinguish in the ordinary way, since the stone will extinguish in small patches and these will not be dark all at the same time; further, the double refraction is small, and on revolving the stone between crossed nicols the extinction is not sharp. Paste stones are sometimes in a state of strain, and display slight, but general, double refraction. Hence the existence of double refraction does not necessarily prove that the stone is real and not an imitation. Stones may be composed of two or more individuals which are related to each other by twinning, in which case each individual would in general extinguish separately. Such individuals would be larger and would extinguish more sharply than the patches of an anomalous stone.
Fig. 28.—Interference of Light.
An examination in convergent light is sometimes of service. An auxiliary lens is placed over the condenser so as to converge the light on to the stone. Light now traverses the stone in different directions; the more oblique the direction the greater the distance traversed in the stone. If it be doubly refractive, in any given direction there will be in general two rays with differing refractive indices and the resulting effect is akin to the well-known phenomenon of Newton’s rings, and is an instance of what is termed interference. It may be mentioned that the interference of light (Fig. 28) explains such common phenomena as the colours of a soap-bubble, the hues of tarnished steel, the tints of a layer of oil floating on water, and so on. Light, after diverging from the stone, comes to focus a little beneath the plane in which the image of the stone is formed. An auxiliary lens must, therefore, be inserted to bring the focal planes together, so that the interference picture may be viewed by means of the same eyepiece.
If a uniaxial crystal be examined along the crystallographic axis in convergent light an interference picture will be seen of the kind illustrated on [Plate III]. The arms of a black cross meet in the centre of the field, which is surrounded by a series of circular rings, coloured in white light. Rotation of the stone about the axis produces no change in the picture.
PLATE III
1. UNIAXIAL
