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 passing 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 passing 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 pass 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 class, technical distinctions made specially for purposes of classification, thus:--a "non-diaphanous" stone is one which is quite opaque, no light of any kind passing 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 passes through its substance, sight is not possible through it; whilst in a "sub-translucent" stone, light passes 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 Lustre, or Colour. ~Lustre.~--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:--