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. EV. Nay, I presume not to so potent a faculty. Many of the slight imperfections of vision are, as I have confessed, merely exaggerations of romantic ideas floating in the memory; and this is not a novel notion, for Plato and other philosophers held it long before our time. _Muscæ volitantes_ are usually, though not always, _substantial_: i.e. depending on _points_ or _fibres_ in the axis of vision, on _congestions_, or _varicose_ states of the vessels of the _choroïd_ or _retina_, or of atoms floating in the _humours_. These specks, which do not appear alike in the eyes of all, and the brilliant beams in the _suffusio scintillans_, so varied and so whimsical, might be readily moulded into human form, by the imagination of an enthusiast, or the feelings of the ghost-seer, who is usually morose and melancholy, in a state of _longing_ for a ghost or a mystery. But when many of the more confirmed illusions are depending on structural disease in the membranes and humours of the eye, I am confident in the resources of our science to relieve, if not to remove. Coleridge indeed has expressed his belief, that by some convulsion of the eye, it may see projected before it part of its own body, easily magnified into the whole by slight imagination. If this be true, the whole mystery of the Death-fetch is unravelled.