Thus, while halos have certain definite radii,
viz. 22 deg. and 46 deg., the radii of coronae vary very considerably;
also, halos are coloured red on the _inside_, whereas coronae are
coloured red on the _outside_ (see HALO).

It has now been firmly established, both experimentally and
mathematically, that coronae are due to diffraction by the minute
particles of moisture and dust suspended in the atmosphere, and the
radii of the rings depend on the size of the diffracting particles. (See
DIFFRACTION OF LIGHT.)

Other meteorological phenomena caused by the diffraction of light
include the _anthelia_, and the chromatic rings seen encircling shadows
thrown on a bank of clouds, mist or fog. These appearances differ from
halos and coronae inasmuch as their centres are at the anti-solar point;
they thus resemble the rainbow. The anthelia (from the Greek [Greek:
anti], opposite, and [Greek: helios], the sun) are coloured red on the
inside, the outside being generally colourless owing to the continued
overlapping of many spectra. The diameter increases with the size of the
globules making up the mist. The chromatic rings seen encircling the
"spectre of the Brocken" are similarly explained.

The blue colour of the sky (q.v.), supernumerary rainbows, and the
gorgeous sunsets observed after intense volcanic disturbances, when the
atmosphere is charged with large quantities of extremely minute dust
particles (e.g. Krakatoa), are also explicable by the diffraction of
light. (See DUST.)

See E. Mascart, _Traite d'optique_ (1899-1903); J. Pernter,
_Meteorologische Optik_ (1902-1905).

In architecture, the term "corona" is used of that part of a cornice
which projects over the bed mould and constitutes the chief protection
to the wall from rain; it is always throated, and its soffit rises
towards the wall. The term is also given to the apse or semicircular
termination of the choir; as at Canterbury in the part called "Becket's
crown." The large circular chandelier suspended in churches, of which
the finest example is that given by Barbarossa to Aix-la-Chapelle, is
often called a corona. The term is also used in botany of the crown-like
appendage at the top of compound flowers, the diminutive being
_coronule_.

CORONACH (a Gaelic word, from _comh_, with, and _ranach_, wailing), the
lamentation or dirge for the dead which accompanied funerals in the
Highlands of Scotland and in Ireland. The more usual term in Ireland i