hy Doiibly Befractmg Crystals, 
List of Absorbing Crystals with One Aocis, 
Colour when its axis is 
Colour when its axis is 
Names of Crystals. 
in the plane of primitive 
perpendicular to that 
polarisation. 
plane. 
Zircon 
Brownish white 
A deeper Brown 
Sapphire 
Yellowish green 
Blue 
Ruby 
Pale yellow 
Bright pink 
Emerald 
Yellowish green 
Bluish green 
Emerald 
Bluish green 
Yellowish green 
Beryl blue * 
Bluish white 
Blue 
Beryl green 
Whitish 
Bluish green 
Beryl yell, green 
Pale yellow 
Pale green 
Rock crystal, almost 
transparent 
Whitish 
Faint brown 
Rock crystal yell. 
Yellowish white 
Yellow 
Amethyst 
Blue 
Pink 
Amethyst 
Greyish white 
Ruby red 
Amethyst 
Reddish yellow 
Ruby red 
Tourmaline 
Greenish white 
Bluish green 
Rubellite 
Reddish white 
Faint red 
Idocrase 
Yellow 
Green 
Mellite 
Yellow 
Bluish white 
Phosp. of lime (lilac) 
Bluish 
Reddish 
olive 
Bluish green 
Yellowish green 
Phosphate of lead 
Bright green 
Orange yellow 
Calcareous spar 
Orange yellow 
Yellowish white 
The property which these crystals possess of absorbing the 
different tints in different positions of the axis, with regard to 
the plane of primitive polarization, does not belong to every 
specimen. There are many crystals of ruby, sapphire, eme- 
rald, &c. which give an ordinary and an extraordinary image 
of the same colour ; and whenever this is the case, they are des- 
titute of the property of absorbing polarised light. These two 
classes of phenomena are indeed invariably connected, and will 
ultimately be found to have the same origin. 
Some of the preceding crystals, such as the Sapphire and the 
Idocrase, exhibit different colours when common light is trans- 
mitted in directions parallel and perpendicular to their axis 
of double refraction. A specimen of sapphire had a deep 
• There are two magnificent crystals of Beryl in the possession of Mr Sander- 
son, which exhibit the phenomena of absorption in a most beautiful manner. 
Both of them absorb a fine deep blue tint when the axis is in the plane of polarisa- 
tion, and a greenish white tint in a rectangular direction. A knowledge of these 
properties of precious stones is necessary, to discover the proper mode of cutting 
them for the purposes of jewellery. 
