A sounding-plate, made of brass, nine inches long, and half a line in
thickness, covered with a layer of water, may be employed to produce
a rainbow in a chamber which admits the sun. On drawing a violin bow
strongly across the plate, so as to produce the greatest possible
intensity of tone, numerous drops of water fly perpendicularly and
laterally upwards. The size of the drops is smaller as the tone is
higher. The inner and outer rainbows are very beautifully seen in these
ascending and descending drops, when the artificial shower is held
opposite to the sun. When the eyes are close to the falling drops,
each eye sees its appropriate rainbow; and four rainbows are perceived
at the same time, particularly if the floor of the room is of a dark
colour. The experiment succeeds best, if, when a finger is placed under
the middle of the plate, and both of the angular points at one side
are supported, the tone is produced at a point of the opposite side,
a fourth of its length from one of its angles. An abundant shower of
drops is thus obtained.