These forces have the same period and direction as the undisturbed luminous vibrations themselves.
St Elmo's Fire.-Luminous discharges from masts, lightning conductors, and other pointed objects are not very infrequent, especially during thunderstorms. On the Sonnblick, where the phenomenon is common, Elster and Geitel (87) have found St Elmo's fire to answer to a discharge sometimes of positive sometimes of negative electricity.
Except for a few species in the New Hebrides, New Caledonia and Fiji, the luminous Elateridae are unknown in the eastern hemisphere.
Beneath the cuticle of these regions are situated the luminous organs, consisting of layers of cells which may be regarded as a specialized portion of the fat-body) Both the male and female fireflies emit light, as well as their larvae and eggs, the egg being luminous even while still in the ovary.
The name "firefly" is often applied also to luminous beetles of the family Lampyridae, to which the well-known glow-worm belongs.