When an excess of blood is determined in the vessels of the eye, either from position or other cause, a luminous arborescent figure is occasionally observed in the field of vision on entering a dark apartment. This, according to Purkinje, is due to pressure on the retina by the distended blood-vessels. A luminous spot is also sometimes observed isochronous with the pulse.
In ourselves, in ordinary health a lambent bluish coloured cloud of light constantly floats before the eyes in a darkened apartment; and there are probably few who would not perceive a greater or less sensation of light on being shut up in profound darkness.
On the spontaneous appearance of light in the field of vision when it is darkened, Müller, the distinguished Prussian physiologist, writes:—"If we observe the field of vision, keeping the eyes closed, it occasionally happens that we perceive not only a certain degree of luminousness, but further, that we discover a more marked glimmering of light, affecting even, in certain cases, the form of circular waves, which are developed from the centre towards the periphery, where they disappear. Sometimes the faint light resembles a nebulosity, spots, and more rarely, in myself, it is reproduced with a certain rhythm. To this spontaneous appearance of light in the eye, which is always very vague, are related the more clearly delineated forms which show themselves at the moment we are about to fall asleep, and which depend upon the influence of the imagination isolating the nebulous glimmerings one from the other, and clothing them with more distinct forms."[58]
The degree to which this sensation of light is produced in health, and the power which the imagination has over it, vary greatly in different individuals.
Müller writes:—
"I had occasion, in 1828, to converse with Göethe upon this subject, which had an equal interest for both of us. Knowing that when I was tranquilly extended in bed, the eyes closed, but not asleep, I frequently perceived figures that I could observe distinctly, he was curious to know what I experienced then: I told him that my will had not any influence either upon the production or the metamorphoses of these figures, and that I never distinguished anything symmetrical, anything that had the character of vegetation. Göethe, on the contrary, was able to appoint at will a theme, which afterwards transformed itself, after a fashion apparently involuntary, but always in obedience to the laws of harmony and symmetry: a difference between two men, of which one possessed the poetical imagination in the highest degree of development, whilst the other devoted his life to the study of reality and of nature.
"Göethe says, 'When I close the eyes, on lowering the head, I imagine that I see a flower in the middle of my visual organ; this flower does not for a moment preserve its form: it is quickly decomposed, and from its interior are born other flowers with coloured or sometimes green petals; these are not natural flowers, but fantastic, nevertheless regular, figures, such as the roses of sculptors. It was impossible for me to regard this creation fixedly, but it continued as long as I wished, without increasing or diminishing. Even when I figured to me a disc charged with various colours, I saw continually borne from the centre towards the circumference, new forms comparable to those that I could perceive in a kaleidoscope."[59]
Illusions arising from the production of the sensation of light, whether by pressure, mental emotion, or a disordered state of the health, have been a most prolific source of ghosts.
Imagine a person suffering from severe grief occasioned by the loss of a friend or relative; or one subject to superstitious terrors. On retiring to rest in a darkened apartment, the attention is attracted and wonder raised by the appearance of a cloud of pale white, or blueish coloured light (the colours which ghosts love to deck themselves in, and which are most readily excited) floating before the eyes. Unacquainted with its nature and source, he is naturally startled, and his superstitious fears are awakened. The imagination next coming into play, the luminous cloud is moulded into the form of the person recently dead, or of the superstitious ideas most prominent in the mind of the individual at the time.