Diamonds And Luxury

Hitherto we have considered only those substances in which a single refracted ray corresponds to a single incident ray. There are, however, many bodies, including many precious stones, which have the property of splitting a single incident ray of light into two refracted rays which are propagated in their substance along paths differing slightly in direction.

In Fig. 21 the ray AB, travelling in air L, is incident upon the surface MN of the stone S at B, where it is split into the two refracted rays BO and BE, inclined to one another at a very small angle OBE, which never exceeds a few degrees.
Bodies which behave towards light in this way are described as being doubly refracting or birefringent, in contradistinction to the singly refracting bodies hitherto considered. Substances exhibiting the phenomenon of double refraction may also be described as optically anisotropic, while those which exhibit single refraction are described as being optically isotropic.
As far as regards the transparency, lustre, colour, and play of colour of a stone—those characters in short which affect the beauty of the stone—it is unimportant whether the light within it is singly or doubly refracted.

The phenomenon of double refraction can be easily observed by the aid of special instruments. The detection of its presence or absence is a valuable aid in identifying and discriminating precious stones in the cut condition. Thus by the aid of an appropriate instrument we can decide whether a certain red stone is a doubly refracting ruby or a singly refracting spinel, two stones which, though very similar in appearance, are very dissimilar in rarity and costliness. It is also possible by this means to distinguish glass imitations, which a.re always singly refracting, from genuine precious stones, which are for the most part doubly refracting.
1ct diamond engagement ring. The kind of refraction, single or double, exhibited by a body is a necessary consequence of the crystalline structure of its substance, and varies in the different crystal systems. All amorphous bodies, together with all those which crystallise in the cubic system, are singly refracting, while all other crystals, without exception, namely, those included in the hexagonal, tetragonal, rhombic, monoclinic, and triclinic systems are doubly refracting. It is thus possible from the behaviour of a stone with respect to the refraction of light to learn whether, on the one hand, it is amorphous or crystallises in the cubic system, or whether, on the other hand, it crystallises in one of the five remaining crystal systems; and this observation can be made on a very small irregular fragment of the mineral. Thus in the example just quoted we know that the singly refracting spinel must crystallise in the cubic system, while the doubly refracting ruby crystallises in one of the remaining five systems, namely, the hexagonal.

Since the observation of the kind of refraction, whether single or double, exhibited by a stone is a step towards determining to which of the crystal systems it belongs to precious stones, aud more¬over is frequently a decisive test of its identity, it is important to be acquainted with the method of making this observation. In the third part of this book, dealing specially with the determination of precious stones, considerable use will be made of this method, and it will also be mentioned under the description of each species of precious stone.

In some substances the phenomenon of double refraction is directly observable, for an object, when viewed through a plate of the substance, will appear double instead of single, as is more usually the case, for example with a plate of glass. Each of the two refracted rays BO and BE (Fig. 21) gives an image of the object; these two images are, as a rule, very close together, but in some few minerals they may be so widely separated as to be both distinctly visible.

In Fig. 22, let MNPQ be a plate of doubly refracting substance with the surface MN parallel to the surface PQ. The incident ray of light AB, striking the surface MN at B, enters the plate and is split up into the two rays BO and BE; these emerge from the surface PQ in the -directions 00' and EE' both parallel to AB. Each of these rays OO1 and EE gives rise to an image of the source of light, and an eye placed at O'E' will see one image along 00 and another along EE. Other conditions being equal, these two images will be the more widely separated the thicker the plate is.

A substance which shows the phenomenon of double refraction to a very marked degree is calcite or Iceland-spar, which on this account is also called doubly refracting spar. If a crystal, or, better still, a transparent cleavage rhombohedron of Iceland-spar is placed over an object, such, for instance, as the page of a book, the letters, w hen viewed through the spar, will appear double.

In calcite the two refracted rays are inclined to each other at a comparatively large angle, much greater than in the majority of other minerals. The greater the angle of separation of the two refracted rays (OBE in Fig. 21) the greater the double refraction of the mineral, and different substances differ considerably from each other in this respect.

The double refraction of the majority of precious stones is not very strong; and as usually only a small thickness of such substances is available for examination, the two images of an object viewed through the stone will be very close together or partly overlap

and then tend to appear as a single image. They would thus, by simply viewing an object through a thin plate, appear to be only singly refracting, whereas in reality they are doubly refracting.

It is possible, however, in such cases to bring about a wider separation of the two images by using a prism instead of a parallel-sided plate of the stone. This is illustrated in Fig. 24, where, as in the case of single refraction (Fig. 18), the rays of white light are decomposed into rays of differently coloured light. The ray of light AB, coming from a small flame at A, on entering the prism is split into two rays travelling in the directions BO and BE. In consequence of dispersion, each of these rajs is separated into its coloured components BOr to BOv and BEr to BEV ; and on passing out at the second surface of the prism NP, are again refracted, and thus emerge still more widely separated. To an eye placed at 0'rE'v, two images of the flame, O'r0'„ and E'rE'v, will be visible close together or partly overlapping. Each image shows the columns of the spectrum of white light as did the image seen through a prism of singly refracting substance; moreover, if thrown on a screen, the red ends, Or and E'r, of both spectra will be nearer the refracting angle of the prism, and the violet ends, 0'v and E'v, further away.

Fig. 25 gives a perspective view of the path of light in a doubly refracting prism, similar to the one given by Fig. 19 in the case of a singly refracting prism. The two faces of the prism MNM'N' and NPN'P are inclined together at the refracting angle MNP and intersect in the refracting edge NN.

A ray, AB, emitted by the centre of the candle flame, A, strikes the face MNM'N of the prism at B, and is refracted along BO and BE. These two refracted rays pass out at the second face NPN'P, and take the directions 00' and EE. To an eye placed at OE, two images of the candle flame will be visible in the directions O'OA" and E'EA". Many precious stones show the two images A" and Ae quite close together, often, indeed, over¬lapping more or less. As was the case with the single image given by a singly refracting prism (Fig. 19), each of the double images has a red margin r and a violet margin v.

Now every facet at the front of a cut transparent gem forms with any facet at the back (provided they are not parallel) a prism ; and through every such pair of facets can be seen, when viewed in the proper direction, an image of a small flame. As a matter of fact, a large number of such images will be seen, since for any one facet at the front of the stone there will be several at the back, each of which may form with the front facet a prism and give rise to an image. The images given by singly refracting stones are single, as in Fig. 19, while doubly refracting stones give two images very close together, as shown in Fig. 25. This difference enables us to distinguish a singly refracting from a doubly refracting stone.

FIG. 25. Path of light through a doubly refracting prism. (Perspective view.) singly refracting, or double if it is doubly refracting. Each image, whether double or single, has originated by refraction through
a prism formed by one of the facets at the back of the stone and the table at the front. The images seen through a doubly refracting stone are shown in Fig. 26a, while those #sen through a singly refracting stone are shown in Fig. 2.6b.

This experiment is best performed in a dark room so that no light other than that from the small flame passes through the stone.
Instead of using a flame, however, any other convenient object may be observed through the stone, and for this purpose a needle may be used. bridal engagement rings. When the needle is placed in the proper position relative to the stone, there will be seen several coloured single images of it in the case of singly refracting stones, while doubly refracting stones will give coloured double images of the needle. Contrary to the previous case, this experiment must be performed in a lighted room.

When a stone thus examined shows unmistakably double images the fact may be regarded as a decisive proof of the doubly refracting nature of the stone ; when, however, single images only are observed the stone cannot be stated to be singly refracting on these

grounds alone, for stones which have only feeble double refraction may give double images so close together, or may be overlapping, that to recognise the double character of such images is a matter of considerable difficulty.

The investigation by the direct method of the kind of refraction possessed by a stone thus requires a certain amount of skill, which is only acquired by practice. On this account the re¬fraction of stones is often investigated by an indirect method, which has the advantage of being applicable to stones with rounded surfaces, and also to small and irregular fragments of material, neither of which could be used with the method of direct observation. Further, very small cut stones are easily examined by the indirect method, while their examination by the direct method would present difficulties.

The instrument used for the indirect observa¬tion of the singly or doubly refracting character of a stone is known as the polariscope. A simple form of this instrument, sufficient for the present purpose, is shown, one-third the actual size, in Fig. 27.
This consists of a wooden box, H, into the cover, pp, of which fits the circular object-carrier, oo; the latter consists of a plate of glass in a brass setting, and may be easily rotated. From the box rises the vertical brass rod, mm, which carries, on the horizontal arm, h, a Nicol's prism, n, constructed of Iceland-spar. This is placed in the same vertical line with the centre of oo, and is capable of being rotated in the arm, h. In the box, H, is fixed, at an angle of 33° with the vertical, a sheet of unsilvered glass, ss, or better still, a large number of thin glass plates arranged in a pile. The box also contains an ordinary mirror, tt, the inclination of which can be varied by means of the wooden wedge, K.

Rays of light from a clear sky enter the open side of the box, as indicated in the figure by the dotted line, and are reflected from the mirror, tt, on to the glass plate, ss, at an angle of 57° with the normal to the plate, whence they are again reflected in a vertical direction through the object-carrier and the Nicol's prism to the eye of the observer.

Ordinary daylight, after reflection from the glass plate, ss, at the particular angle mentioned above, becomes endowed with special properties, and is said to be polarised. n other words, the rays of ordinary light which strike the plate, ss, are reflected from it as rays of polarised light, and as such reach the Nicol's prism, n. On rotating the Nicol's prism, it

will be found that in certain positions it does not allow the light reflected from ss to pass through, and in these positions the field of view becomes dark, while in other positions it is light. On turning the NicoFs prism through a complete revolution, that is 360°, it will be observed that there are four gradual changes from maximum lightness to maximum darkness or vice versd ; the four positions of maximum lightness and maximum darkness being separated by angles of 90°. If the NicoFs prism be turned into one of the two positions in which the field of view has maximum darkness, the polariscope will afford a means whereby singly and doubly refracting stones can be distinguished from each other with ease and certainty.

The different behaviour of singly and doubly refracting substances when examined with the polariscope is as follows : When a singly refracting substance, such as a piece of glass, is placed on the object-carrier, no, and observed through the NicoFs prism, the whole of the field of view will be dark and will remain dark during the rotation of the object-carrier. It should be mentioned here that in this, as well as in all other observations, it is advisable to shade the side light from the object with the hand, or, better still, by means of a tube of black paper placed on the object-carrier and round the object; otherwise light will reach the eye which has been reflected from the surface of the stone without passing through it.

When a doubly refracting body is examined in the same way, it is found that in certain positions the portion of the field which it occupies becomes light. This is due to the fact that the polarised light, which before was unable to pass through the NicoFs prism, becomes so modified by its passage through the doubly refracting substance that it is now capable of passing through the NicoFs prism, when it will reach the eye of the observer. As the object is turned round through 360°, there will be eight changes from maximum lightness to maximum darkness or vice versd ; there being four positions of the stone in which the lightness is a maximum, and four in which there is maximum darkness, an interval of 45° lying between each. Through the complete rotation of the object, however, the portion of the field not occupied by it remains dark so long as the NicoFs prism is undisturbed.

There is thus an essential and important difference in the behaviour of singly and doubly refracting stones when examined in polarised light. A singly refracting stone remains dark in the dark field of the polariscope, while a doubly refracting stone changes from light to dark as it is rotated with the object-carrier.

Even in this method, however, there are certain liabilities to error which must be carefully avoided. In all doubly refracting substances the strength of the double refraction is not the same in all directions. Thus the two images of a flame or needle seen through a doubly refracting stone will be further apart when viewed in some directions than in others, while in certain directions a single image only is to be seen. The substance is therefore, along these particular directions, not doubly refracting but singly refracting.

Those directions in a doubly refracting body along which there is only single refraction are known as optic axes. All doubly refracting stones can be grouped into two classes: the one containing stones having one optic axis, described as being optically uniaxial; and the other containing stones having two optic axes, and described as being optically biaxial. The optic axes of any given substance are closely connected both in number and direction with its crystalline form. Thus all hexagonal and tetragonal crystals are uniaxial, and the optic axis of these crystals coincides in direction with the principal crystallographic axis. All rhombic, monoclinic, and triclinic crystals are biaxial, and in the case of rhombic and monoclinic crystals definite relations exist between certain crystallographic and optical directions. Crystals belonging to the remaining system, namely the cubic, are, as mentioned above, optically isotropic, that is, singly refracting.