G2 PHYSICAL PROPERTIES OF MINERALS. 
Hair-brown, reddish-brown, chestnut-brown, yellowish-brown, 
pinchbeck-brown, wood-brown. 
A play of colors—this expression is used when several pris- 
matic colors appear in rapid succession on turning the mineral. 
‘The diamond is a striking example ; also precious "opal. 
Change of colors—when the colors change slowly on turning 
in different positions, as in labradorite. 
Opalescence—when there is a milky or pearly reflection 
from the interior of a specimen, as in some opals, and in cat’s 
eye. 
Lridescence—when prismatic colors are seen within a crystal, 
it is the effect of fracture, and is common in quartz. 
Zarnish—when the surface colors differ from the interior; 
it is the result of exposure. The tarnish is described as wrised 
when it has the hues of the rainbow. 
2. Dichroism, Tichroism.—Some crystals, under each of the 
systems excepting the isometric, have the property of present- 
ing different colors by transmitted light in different directions. 
The property is called dichroism when these colors are seen in 
two directions, and trichroism (or pleochroism) if seen in three 
directions. The colors are always the same in the direction 
of equal axes and often unlike in the direction of unequal 
axes. As dimetric and hexagonal crystals have the lateral axes 
equal they can present different colors only in two directions, 
the vertical and lateral; while all crystals that are optically 
biaxial may be trichroic. 
The mineral iolite is a noted example, and received the name 
dichrowte on account of this property. Transparent colored 
crystals of tourmaline, topaz, epidote, mica, diaspore, and many 
other species exhibit it. Tourmaline cr ystals, when transpar- 
ent or translucent transverse to the prism, are opaque in the 
direction of the vertical axis; and so also are thick crystals of 
mica. Colored varieties of hornblende are dichroic, while 
those of the related mineral, pyroxene, are not so. 
This quality is best observed by means of polarized light. On 
examining a mineral with a tourmaline plate, or Nicol prism, the 
two colors in a dichroic mineral are successively seen as the 
tourmaline or Nicol is revolved; and if there is no dichroism 
there is no change of color. A small instrument, containing a 
prism of calcite, has been constructed for showing the dichro- 
ism, called the dichroscope. On looking through it at a di- 
Glnoie crystal, the aperture against the crystal appears double, 
owing to the double refraction of the calcite, one image being 
made by the ordinary ray and the other by the extraordinary 
