In many cases of disordered sensibility of the retina, it is influenced by the minute _villi_ or vessels in the _tunics_ of the eye. In the case of exhausted energy of this retina, usually accompanied by night-blindness, where there is no vision but in a strong light, floating specks termed _muscæ volitantes_ often become so numerous, as to impart a notion of films floating in the watery humour of the eye, or before the _cornea_. It is a curious question, in what portion of the retina the _spectra_ of _muscæ volitantes_ are excited. They appear in or near the axis of vision; but as they do not interrupt the _visual rays_ from _material objects_, it is possible they may arise on that spot considered to be destitute of vision, with regard to _external_ impression. Or they may be produced by detached parts only of the objects, which impinge on the retina, reaching the brain. If the integrity of certain of its fibres, which by converging form the optic nerve, be destroyed, distorted or imperfect objects will be presented. This speck may be a _musca volitans_. ASTR. The original impressions in all cases are, I presume, from _without_: how is the _internally_ excited idea presented as a prominent image _before_ the eye? EV. That form of disordered vision to which I allude, occurring so often in nervous persons, or resulting from close application to study, does not often appear to depend on a _turgid_ condition of the vessels of the _choroïd coat_ or _retina_. It is usually relieved more by _tonics_ than by depletion; and very strange illusions of sight will sometimes be produced merely by depressing medicines, especially the preparations of antimony. Yet these dark specks appear to be floating _before_, and often at some distance _withoutside_ the eye. Therefore we may believe that excited images or _more perfect forms_ may also appear before the retina, _palpable_. Between the first impression and its recurrence, a long period may have passed (memory being unlimited); and it is sufficient that one sole idea be excited to produce a succession; as a _spark_ of fire will ignite a _train_ of gunpowder; or as an electric spark will discharge a whole battery. In the curious case of _photopsia_, or _suffusio scintillans_, we have a series of illusive spectra, in the forms of “lucid points,” and “yellow flames,” and “fiery veils,” and “rings of light.” In some cases of ophthalmia, and in acute inflammation of the brain, the candles and other bright objects in the chamber will look like blood. Beguelin, as we read in the “Berlin Memoirs,” by straining his eyes on a book, always saw the letters red.