ced by two reflexions and two refractions. The colours
of the secondary bow are much fainter than those of the primary, because
they undergo two reflexions instead of one.

There is something very wonderful in the rapidity and perfection with
which these natural prisms, the falling drops of rain, produce these
effects. In the inconceivably short space of time occupied by a drop
falling through those parts of the sky which form the proper angles with
the sun's rays and the eye of the observer, the light enters the surface
of the drop, undergoes within it one or two reflexions, two refractions
and decompositions, and has reached the eye; and all this is done in a
portion of time too small for the drop to have fallen through a space
which we have the means of measuring.

It will be understood, that since the eyes of different observers cannot
be in precisely the same place at the same time, no two observers can see
the _same_ rainbow; that is to say, the bow produced by one set of drops
to the eye of one observer is produced by another set of drops to the eye
of another observer.

A rainbow can never be greater than a semicircle, unless the spectator is
on elevated ground; for if it were greater than a semicircle the centre
of the bow would be above the horizon, while the sun, which must be in a
line drawn through that centre and the eye of the observer, would be
below the horizon: but in such a case, the sun could not shine on the
drops of rain, and consequently there could be no rainbow.

When the rain cloud is of small extent only a portion of a bow is
visible; when the cloud overspreads a large part of the sky a perfect bow
appears. Sometimes the bow may be traced across a portion of blue sky,
or it may appear to rest on the ground. In the former case, there are
vapours in the air too thin to be seen, but sufficient to refract and
reflect the rays of light; in the latter, the drops of rain, adhering to
the grass and foliage, produce the same effect. A coloured bow, similar
to that produced by rain, is sometimes seen in the spray of a fountain or
of a water-fall, and also in mists that lie low upon the ground.

In mountainous and stormy regions rainbows are often seen to great
advantage. In the islands off the Irish coast the author of "Letters
from the Irish Islands," describes the rainbow of winter "as gradually
advancing before the lowering clouds, sweeping with majestic stride
across the troubled ocean, th