Brought Italian rock-crystal cutters from Milan to take control of the crystal and glass-cutting works he had established at Prague.
Air, flint, glass, rock-crystal, calcareous spar were examined, but without effect.
Clear transparent rock-crystal is used for optical purposes and spectacle lenses.
Further, these varieties may be of almost any colour, whereas transparent crystals have only a limited range of colour, being either colourless (rock-crystal), violet (amethyst), brown (smoky quartz) or yellow (citrine).
A few magnificent specimens of rock-crystal twinned according to this law have been found at La Gardette in Isere, and in Japan they are somewhat abundant.
There appears, however, to be some objection to the use of iridio-platinum for weights, as, owing to its great density (Î”=21.57), the slightest abrasion will make an appreciable difference in a weight; sometimes, therefore, quartz or rock-crystal is used; but to this also there is some objection, as owing to its low density (Î”=2.65) there is a large exposed surface of the mass.
He fell upon a most ingenious plan of doubling the amount of double refraction of a prism by using two prisms of rock-crystal, so cut out of the solid as to give each the same quantity of double refraction, and yet to double the quantity in the effect produced.
It is contained in a rock-crystal shrine, encased in silver, and is vested in full pontifical robes blazing with jewels.