There is a story in Voltaire, that the Duke of Florence threw the dice with a field-officer of his enemy. The spots on the dice seemed, to his excited brain, like drops of blood: he instantly ordered a retreat of his army. And this is not wonderful; it is but excited sensibility, of which many analogies indeed may be artificially produced, as the flash of light from the pricking of the retina with a fine needle, and the beautiful iris which is formed by pressure on the globe of the eye. In the very interesting history of the prisoner in the dungeon of the Chatelet at Paris, the phosphorescence of the eye was itself the source of light, in this instance so powerful as to enable the prisoner to discern the mice that came around him to pick up the crumbs, although the cell was pitchy dark to others.
There are many curious illusions resulting from over-straining or over-excitement of the eye.
Dr. Brewster, in the Edinburgh Journal of Science, vol. iii. says, “If in a fine dark night we unexpectedly obtain a glimpse of any object, either in motion or at rest, we are naturally anxious to ascertain what it is, and our curiosity calls forth all our powers of vision. Excited by a feeble illumination, the retina is not capable of affording a permanent vision of the object; and while we are straining our eye to discover its nature, it will entirely disappear, and afterwards re-appear and vanish alternately.”
A friend of Buffon had been watching the progress of an eclipse through a very minute aperture. For three weeks after this there was a perfect spectrum of the lucid spot marked on every object on which he fixed his eyes.
Dr. Brewster had been making protracted experiments on some brilliant object, and for several hours after this a dark spectrum, associated with intense pain, floated constantly before his eye.
In the third volume of his Physiology, Dr. Bostock thus concludes the account of his own ocular spectra: “It appeared as if a number of objects, principally human faces or figures, on a small scale, were placed before me, and gradually removed, like a succession of medallions. They were all of the same size, and appeared to be all situated at the same distance from the face. After one had been seen for a few minutes, it became fainter, and then another which was more vivid seemed to be laid upon it, or substituted in its place, which in its turn was superseded by a new appearance.”
Coloured vision may arise from permanent defect or from acute disorder: from some peculiar refraction of a ray of light on the lens of the eye, or by the optical laws of the accidental colours.
The ray of white light consists of the three prismatic or primitive colours. Now, if the eye is fatigued by one of these colours, or it be lost, mechanically or physiologically, the impression of two only will remain, and this accidental or complementary colour is composed of the two remaining constituents of the white ray. Thus, if the eye has been strained on a red colour, it is insensible to this, but perceives the blue and the yellow, the combination of which is green. So, if we look long on a green spot, and then fix the eye on white paper, the spectrum will be of light red. A violet spot will become yellow; a blue spot orange-red: a black spot will entirely disappear on a white ground, for it has no complementary colour; but it appears white on a dark ground, as a white spot will change to black.
By this law I may explain the impression made by black letters on the red ground of a play-bill, which appear blue. The accidental colour of orange-red is blue; that of black is white. By looking on this, the black letter first becomes white, and the accidental colour of the red—blue, is transferred to the white ground of the letters.
Astr. Then, as D’Agessau recommended the parliament of Paris to leave the demoniac of our times to the physician and not the divine, you would delegate the management of all those, to whom the mysterious world of shadows is unfolded, to the sapient leech with his phials and his lancet.