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and then another which was more vivid seemed to be laid upon it, or substituted in its place, which in its turn was superseded by a new appearance.”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.