The tail is laterally compressed, nearly naked, and scaly.
In Pteromys the tail is cylindrical and comparatively thin, while in Sciuropterus it is broad, flat and laterally expanded, so as to compensate for the absence of the interfemoral membrane by acting as a supplementary parachute.
The light actually emitted laterally is thus the same as would be caused by forces exactly the opposite of these acting on the medium otherwise free from disturbance, and it only remains to see what the effect of such force would be.
The principle of energy makes it clear that the light emitted laterally is not a new creation, but only diverted from the main stream.
Wilson supposes that by the fall to the ground of a preponderance of negatively charged rain the air above the shower has a higher positive potential than elsewhere at the same level, thus leading to large conduction currents laterally in the highly conducting upper layers.