Phosphorescence.—The
emission of light by a substance after being heated, rubbed or exposed
to light or other forms of radiant energy. So called from the marked
presence of the phenomenon in phosphorus.
Plane of Incidence.—The plane containing an incident ray of light and the normal to the surface on which the light falls.
Plutonic Rocks.—Rocks which have consolidated at great depths and hence under great pressure.
Polysynthetic Twinning.—Repeated twinning parallel to one plane.
Pseudomorph.—Having
the external form of another substance; thus, Turquois which has no
external crystalline form of its own may be found in the form of a
crystal of Felspar through a gradual decomposition of the Felspar with
formation of Turquois.
Re-entering Angle.—A hollow angle contained between two planes, similar to the under surface of the roof of a house.
Reflection of Light.—The
turning back of light when it strikes a polished surface. The angles of
incidence and of reflection are equal and in the same plane.
Refraction.—The retardation or acceleration of light waves on entering a more or less dense medium, bending the ray.
Refraction, Single.—The simple bending of the light ray that occurs equally in all directions in a transparent amorphous or cubic substance.
Refraction, Double.—The bending of a light ray in two directions that occurs in all transparent substances not either amorphous or cubic.
Rhombohedra.—Solid
figures contained by six planes, opposite pairs of planes being
parallel, and the three upper planes all inclined to the vertical axis
at equal angles. The form may be conceived by the extension of
alternate faces of the double hexagonal pyramid, six faces instead of
twelve being present; were the other six faces of a similar pyramid
developed, the two forms would be positive and negative rhombohedra of
the same order.
