rticles or minute irregularities of structure. Some fibrous
specimens of tremolite and gypsum are translucent in the direction of
the fibres, and practically opaque in a transverse direction. Coloured
transparent objects vary in shade and hue according to their size; thus,
a conical glass filled with a red liquid commonly appears yellow at the
bottom, varying through orange up to red at the upper part. A coloured
powder is usually of a much lighter tint than the substance in bulk, as
the light is reflected back after transmission through only a few thin
layers. For the same reason the powders of transparent substances are
opaque.

Polished bodies, whether opaque or transparent, when illuminated with
white light and viewed at the proper angle, reflect the incident light
regularly and appear white, without showing much of their distinctive
colours.

Some bodies reflect light of one colour and transmit that of another;
such bodies nearly always possess the properties of _selective_ or
_metallic reflection_ and _anomalous dispersion_. Most of the coal-tar
dyes belong to this category. Solid eosin, for example, reflects a
yellowish-green and transmits a red light. Gold appears yellow under
ordinary circumstances, but if the light is reflected many times from
the surface it appears a ruby colour. On the other hand, a powerful beam
of light transmitted through a thin gold-leaf appears green.

Some solutions exhibit the curious phenomenon of _dichromatism_ (from
[Greek: di-], double, and [Greek: chroma], colour), that is, they appear
of one colour when viewed in strata of moderate thickness, but of a
different colour in greater thicknesses (see Absorption of Light).

The blue colour of the sky (q.v.) has been explained by Lord Rayleigh as
due to the scattering of light by small suspended particles and air
molecules, which is most effective in the case of the shorter waves
(blue). J. Tyndall produced similar effects in the laboratory. The green
colour of sea-water near the shore is also due to a scattering of light.

The colours of bodies which are gradually heated to white incandescence
occur in the order--red, orange, yellow, white. This is because the
longer waves of red light are first emitted, then the yellow as well, so
that orange results, then so much green that the total effect is yellow,
and lastly all the colours, compounding to produce white. Fluorescent
bodies have the power of converting light of one colour int