Altitude is said not only to influence the quality of the sleep but the dreams as well. Thus, one who sleeps several thousand feet above sea level is said to have a pleasant thrill as sleep comes on, followed by pleasant dreams. If the sleeping chamber is very high, as on mountain tops, the sleep is fitful and disturbed by weird

dream-phantoms. One who sleeps in poorly ventilated rooms, or in marshy regions, may be troubled by nightmare. As one ascends from sea level, the air becomes rarefied, and physical changes occur, as a lowering of blood pressure; such will account for the points first mentioned, though, of course, they merely instigate the dream or colour it. Disturbing dreams instigated by unhygienic surroundings are usually due to respiratory difficulties. One investigator, Boerner, has succeeded in producing nightmare by lying on the face and closing up the openings of the respiratory organs.

We can best consider preventative dreams by noting the various effects of stimuli on the senses, as on sight, hearing, touch, smell, and taste.

Though the eyelids are closed in sleep most dreams are visual. Barely do we dream words, and almost never do we dream smells and tastes. Dreams are, because of their visual nature, called visions by many. The term vision was used by Macrobius, an ancient writer, who divided dreams into various classes, as a vision of the night in which one saw something in the same way that it later came to pass. We have various reasons to offer in explanation of the visual nature of dreams. For one thing, it is not absolutely necessary that actual vision be employed in order for an image to be produced in the mind; we have accumulated such a vast store of memory pictures that, even though the eyes are closed, a thought will give rise to a corresponding memory image. Again, it is the nature of dreams to deal with images; as stated, dreams are rarely wordy, nor do they often deal with smells and tastes; consequently what the dream wishes to "say" must, as a rule, be made visual. Even our conversations in dreams rarely are accompanied with a feeling that the words spoken have been heard; there is a sort of transference of ideas, a conversation in the imagination, the dreamer, of course, supplying all the words that seem to proceed from the various characters. It is well to note that visual dreams have various characteristics. For instance, the objects seen are in more or less constant motion, something in the nature of a motion picture. Vivid colours are generally absent, which may be due to a failure to note colours, or absence of pronounced colours in the dream. The usual colour seen is grey, or a single other subdued tint. Should the dream be described as rich in colours, especially vivid ones, the cause may be headache, or drugs. Dreams said to be marked by many colours are apt to have had the colours supplied by waking thought.

On closing the eyes at night, as well as at other times, it is often possible to catch fleeting glimpses of various, more or less indistinct pictures on the curtains of the eyelids; also, spots of different colours, on a dark background, sometimes dull, sometimes distinct, and which are constantly changing; they may assume giant proportions, at other times they shrink almost to nothingness; they may also move slowly about the dark background, or they may dance about with the rapidity of a whirling dervish. The phenomenon has been attributed to various causes, as the circulation of the blood in the retinal blood vessels, and pressure on the eyeballs by the closed lids, which in turn produces a mechanical stimulation of the optic nerves. According to this view, the spots are due to central (brain), and peripheral (eye) activities. We have before alluded to these visions in discussing the hypnagogic state.

Henri Bergson1 states that the principal material of which we shape our dreams are the spots which appear when the eyes are closed. "One reads, for example, a newspaper; that is the dream. One awakens and there remains of the newspaper, whose definite outlines are erased, only a white spot with black marks here and there; that is reality. Or our dream takes us upon the open sea - round about us the ocean spreads its waves of yellowish grey with here and there a crown of white foam. On awakening, it is all lost in a great spot, half yellow and half grey, sown with brilliant points. The spot was there, the brilliant points were there. There was really presented to our perceptions, in sleep, a visual dust, and it was this dust which served for the fabrication of our dreams." But, as he says, these impressions, as well as impressions from the ears, the body, outside sources, are merely the material for dreams; they do not produce dreams, the latter being the office of the memory. Doubtless, these spots may influence dreams to some extent in many cases; should the eyes be strained, or headache be present, their influence may be marked. However, countless people are unaware of their existence, and are probably influenced in their dreams little or none by them.

Light falling on the closed eyelids may instigate dreams of being in a burning building, or a conflagration of some sort. The most common light which influences dreams is that of the sun. Weygandt dreamt that he was looking at "living pictures," under magnesium light, and awoke to find the sun just coming from behind the clouds. A lady states that she dreamt of being at a motion picture studio. In order to test her photographing abilities, she and the other applicants for positions as actresses were exposed to a very strong light. The light was so strong as to temporarily blind her. She went into a dark room to recover her sight, and returned for the test in a few moments, her sight having been restored. She awoke shortly after, and found the sun shining on her eyes. Incidentally it might be mentioned that the sun instigated this particular dream because of a conversation the lady had a few days before relative to the failure of many good looking people to photograph well before the motion picture camera. Doubtless she had wondered if she would fail, if she were to make a test, and the sun awakened this thought. Hammond mentions the ease of a man who dreamed that he was in heaven and was dazzled by its brilliancy; he awoke to find that the bright light of a fire was falling on his face.