mists of light, and so on, due to changes of blood-pressure in the retina, only manifest themselves clearly when the eyes are closed and the more powerful effect of the external stimulus cut off.  These subjective spectra come into prominence in the sleepy condition, giving rise to what M. Maury calls “hallucinations hypnagogiques,” and which he regards (after Gruithuisen) as the chaos out of which the dream-cosmos is evolved.[83] They are pretty certainly the starting-point in those picturesque dreams in which figure a number of bright objects, such as beautiful birds, butterflies, flowers, or angels.

That the visual images of our sleep do often involve the peripheral regions of the organ of sight, seems to be proved by the singular fact that they sometimes persist after waking.  Spinoza and Jean Paul Richter both experienced this survival of dream-images.  Still more pertinent is the fact that the effects of retinal fatigue are producible by dream-images.  The physiologist Gruithuisen had a dream, in which the principal feature was a violet flame, and which left behind it, after waking, for an appreciable duration, a complementary image of a yellow spot.[84]

Subjective auditory sensations appear to be much less frequent causes of dream-illusions than corresponding visual sensations.  Yet the rushing, roaring sound caused by the circulation of the blood in the ear is, probably, a not uncommon starting-point in dreams.  With respect to subjective sensations of smell and taste, there is little to be said.  On the other hand, subjective sensations due to varying conditions in the skin are a very frequent exciting cause of dreams.  Variations in the state of tension of the skin, brought about by alteration of position, changes in the character of the circulation, the irradiation of heat to the skin or the loss of the same, chemical changes,—­these are known to give rise to a number of familiar sensations, including those of tickling, itching, burning, creeping, and so on; and the effects of these sensations are distinctly traceable in our dreams.  For example, the exposure of a part of the body through a loss of the bed-clothes is a frequent excitant of distressing dreams.  A cold foot suggests that the sleeper is walking over snow or ice.  On the other hand, if the cold foot happens to touch a warm part of the body, the dream-fancy constructs images of walking on burning lava, and so on.

These sensations of the skin naturally conduct us to the organic sensations as a whole; that is to say, the feelings connected with the varying condition of the bodily organs.  These include the feelings which arise in connection with the processes of digestion, respiration, and circulation, and the condition of various organs according to their state of nutrition, etc.  During our waking life these organic feelings coalesce for the most part, forming as the “vital sense” an obscure background for our clear discriminative consciousness, and only come