hallucinations.

_Dream-Illusions._

By dream-illusions I mean those dreams which set out from some
peripheral nervous stimulation, internal or external. That the organic
processes of digestion, respiration, etc., act as stimuli to the centres
in sleep is well known. Thus, David Hartley assigns as the second great
source of dreams "states of the body."[79] But it is not so well known
to what an extent our dreams may be influenced by stimuli acting on the
exterior sense-organs. Let us first glance at the action of such
external stimuli.

_Action of External Stimuli._

During sleep the eyes are closed, and consequently the action of
external light on the retina impeded. Yet it is found that even under
these circumstances any very bright light suddenly introduced is capable
of stimulating the optic fibres, and of affecting consciousness. The
most common form of this is the effect of bright moonlight, and of the
early sun's rays. Krauss tells a funny story of his having once, when
twenty-six years old, caught himself, on waking, in the act of
stretching out his arms towards what his dream-fancy had pictured as the
image of his mistress. When fully awake, this image resolved itself into
the full moon.[80] It is not improbable, as Radestock remarks, that the
rays of the sun or moon are answerable for many of the dreams of
celestial glory which persons of a highly religious temperament are said
to experience.

External sounds, when not sufficient to rouse the sleeper, easily
incorporate themselves into his dreams. The ticking of a watch, the
stroke of a clock, the hum of an insect, the song of a bird, the patter
of rain, are common stimuli to the dream-phantasy. M. Alf. Maury tells
us, in his interesting account of the series of experiments to which he
submitted himself in order to ascertain the result of external
stimulation on the mind during sleep, that when a pair of tweezers was
made to vibrate near his ear, he dreamt of bells, the tocsin, and the
events of June, 1848.[81] Most of us, probably, have gone through the
experience of impolitely falling asleep when some one was reading to us,
and of having dream-images suggested by the sounds that were still
indistinctly heard. Scherner gives an amusing case of a youth who was
permitted to whisper his name into the ear of his obdurate mistress,
the consequence of which was that the lady contracted a habit of
dreaming about him, which led to a felicitous change of fee