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 feeling on her part.[82]

The two lower senses, smell and taste, seem to play a less-important part in the production of dream-illusions.  Radestock says that the odour of flowers in a room easily leads to visual images of hot-houses, perfumery shops, and so on; and it is probable that the contents of the mouth may occasionally act as a stimulus to the organ of taste, and so give rise to corresponding dreams.  As Radestock observes, these lower sensations do not commonly make known their quality to the sleeper’s mind.  They become transformed at once into visual, instead of into olfactory or gustatory percepts.  That is to say, the dreamer does not imagine himself smelling or tasting, but seeing an object.