*5 
by doubly refracting crystals. 
The property which I have now described as belonging to 
Calcareous spar, I have found in other twelve crystals with 
One axis. The colour of the ordinary and extraordinary 
images, or of the absorbed pencils m and n , is shown in the 
following table. 
List of Absorbing Crystals with One Axis • 
Names of Crystals. 
Colour when its axis is 
in the plane of primi- 
Colour when its axis 
is perpendicular to 
Zircon 
tive polarisation. 
Brownish white 
that plane. 
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, al- 
most 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 
Bluish green 
Bright green 
Yellowish green 
Orange yellow 
Phosphate of lead 
Calcareous spar 
Orange yellow 
Yellowish white 
