Part 2 (1/2)
A well-known simple experiment in physics shows this clearly. A mark on a card or paper is viewed through a piece of double-refracting spar (Iceland spar or clear calcite), when the mark is doubled and two appear. On rotating this rhomb of spar, one of these marks is seen to revolve round the other, which remains stationary, the moving mark pa.s.sing further from the centre in places. When the spar is cut and used in a certain direction, we see but one mark, and such a position is called its optical axis.
_Polarisation_ is when certain crystals possessing double refraction have the power of changing light, giving it the appearance of poles which have different properties, and the polariscope is an instrument in which are placed pieces of double-refracting (Iceland) spar, so that all light pa.s.sing through will be polarised.
Since only crystals possessing the property of double refraction show polarisation, it follows that those of the 1st, or cubic system--in which the diamond stands a prominent example--fail to become polarised, so that when such a stone is placed in the polariscope and rotated, it fails _at every point_ to transmit light, which a double-refracting gem allows to pa.s.s except when its optical axis is placed in the axis of the polariscope, but this will be dealt with more fully when the methods of testing the stones come to be considered.
_Diaphaneity_, or the power of transmitting light:--some rather fine trade distinctions are drawn between the stones in this cla.s.s, technical distinctions made specially for purposes of cla.s.sification, thus:--a ”non-diaphanous” stone is one which is quite opaque, no light of any kind pa.s.sing through its substance; a ”diaphanous” stone is one which is altogether transparent; ”semi-diaphanous” means one not altogether transparent, and sometimes called ”sub-transparent.” A ”translucent”
stone is one in which, though light pa.s.ses through its substance, sight is not possible through it; whilst in a ”sub-translucent” stone, light pa.s.ses through it, but only in a small degree.
The second physical property of light is seen in those stones which owe their beauty or value to REFLECTION: this again may be dependent on l.u.s.tre, or Colour.
~l.u.s.tre.~--This is an important characteristic due to reflection, and of which there are six varieties:--([alpha]) adamantine (which some authorities, experts and merchants subdivide as detailed below); ([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon]) vitreous; ([zeta]) metallic. These may be described:--
([alpha]) Adamantine, or the peculiar l.u.s.tre of the diamond, so called from the l.u.s.tre of adamantine spar, which is a form of corundum (as is emery) with a diamond-like l.u.s.tre, the hard powder of which is used in polis.h.i.+ng diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3}) and is, roughly, four times as heavy as water. The l.u.s.tre of this is the true ”adamantine,” or diamond, brilliancy, and the other and impure divisions of this particular l.u.s.tre are: _splendent_, when objects are reflected perfectly, but of a lower scale of perfection than the true ”adamantine” standard, which is absolutely flawless. When still lower, and the reflection, though maybe fairly good, is somewhat ”fuzzy,” or is confused or out of focus, it is then merely _s.h.i.+ning_; when still less distinct, and no trace of actual reflection is possible (by which is meant that no object can be reproduced in any way to define it, as it could be defined in the reflection from still water or the surface of a mirror, even though imperfectly) the stone is then said to _glint_ or _glisten_. When too low in the scale even to glisten, merely showing a feeble l.u.s.tre now and again as the light is reflected from its surface in points which vary with the angle of light, the stone is then said to be _glimmering_. Below this, the definitions of l.u.s.tre do not go, as such stones are said to be _l.u.s.treless_.
([beta]) Pearly, as its name implies, is the l.u.s.tre of a pearl.
([gamma]) Silky, possessing the sheen of silk, hence its name.
([delta]) Resinous, also explanatory in its name; amber and the like come in this variety.
([epsilon]) Vitreous. This also explains itself, being of the l.u.s.tre of gla.s.s, quartz, etc.; some experts subdividing this for greater defining accuracy into the ”sub-vitreous” or lower type, for all but perfect specimens.
([zeta]) Metallic or Sub-metallic. The former when the l.u.s.tre is perfect as in gold; the latter when the stones possess the less true l.u.s.tre of copper.
~Colour.~--Colour is an effect entirely dependent upon light, for in the total absence of light, such as in black darkness, objects are altogether invisible to the normal human eye. In daylight, also, certain objects reflect so few vibrations of light, or none, that they appear grey, black, or jet-black; whilst those which reflect all the rays of which light is composed, and in the same number of vibrations, appear white. Between these two extremes of _none_ and _all_ we find a wonderful play and variety of colour, as some gems allow the red rays only to pa.s.s and therefore appear red; others allow the blue rays only and these appear blue, and so on, through all the shades, combinations and varieties of the colours of which light is composed, as revealed by the prism. But this is so important a matter that it demands a chapter to itself.
The third physical property of light, PHOSPh.o.r.eSCENCE, is the property possessed by certain gems and minerals of becoming phosph.o.r.escent on being rubbed, or on having their temperature raised by this or other means.
It is difficult to say exactly whether this is due to the heat, the friction, or to electricity. Perhaps two or all of these may be the cause, for electricity is developed in some gems--such as the topaz--by heat, and heat by electricity, and phosph.o.r.escence developed by both.
For example, if we rub together some pulverised fluorspar in the dark, or raise its temperature by the direct application of heat, such as from a hot or warm iron, or a heated wire, we at once obtain excellent phosph.o.r.escence. Common quartz, rubbed against a second piece of the same quartz in the dark, becomes highly phosph.o.r.escent. Certain gems, also, when merely exposed to light--sunlight for preference--then taken into a darkened room, will glow for a short time. The diamond is one of the best examples of this kind of phosph.o.r.escence, for if exposed to sunlight for a while, then covered and rapidly taken into black darkness, it will emit a curious phosph.o.r.escent glow for from one to ten seconds; the purer the stone, the longer, clearer and brighter the result.
CHAPTER VI.
PHYSICAL PROPERTIES.
D--COLOUR.
Colour is one of the most wonderful effects in nature. It is an attribute of light and is not a part of the object which appears to be coloured; though all objects, by their chemical or physical composition, determine the number and variety of vibrations pa.s.sed on or returned to the eye, thus fixing their own individual colours.
We have also seen that if an _equal_ light-beam becomes obstructed in its pa.s.sage by some substance which is denser than atmospheric air, it will become altered in its direction by refraction or reflection, and polarised, each side or pole having different properties.
Polarised light cannot be made again to pa.s.s in a certain direction through the crystal which has polarised it; nor can it again be reflected at a particular angle; so that in double-refracting crystals, these two poles, or polarised beams, are different in colour, some stones being opaque to one beam but not to the other, whilst some are opaque to both.
This curious phenomenon, with this brief, though somewhat technical explanation, shows the cause of many of the great charms in precious stones, for when viewed at one angle they appear of a definite colour, whilst at another angle they are just as decided in their colour, which is then entirely different; and as these angles change as the eye glances on various facets, the stone a.s.sumes a marvellous wealth of the most brilliant and intense colour of kaleidoscopic variety, even in a stone which may itself be absolutely clear or colourless to ordinary light.
Such an effect is called pleochroism, and crystals which show variations in their colour when viewed from different angles, or by transmitted light, are called pleochroic, or pleochromatic--from two Greek words signifying ”to colour more.” To aid in the examination of this wonderfully beautiful property possessed by precious stones, a little instrument has been invented called the dichroscope, its name showing its Greek derivation, and meaning--”to see colour twice” (twice, colour, to see). It is often a part of a polariscope; frequently a part also of the polarising attachment to the microscope, and is so simple and ingenious as to deserve detailed explanation.