[105] On the Magnetization of Light, and the Illumination of Magnetic Lines of Force: by Michael Faraday, D.C.L., F.R.S.—Philosophical Transactions, vol. cxxxvii.—The following remarks are to the point of doubt referred to in the text.—“The magnetic forces do not act on the ray of light directly and without the intervention of matter, but through the mediation of the substance in which they and the ray have a simultaneous existence; the substances and the forces giving to and receiving from each other the power of acting on the light. This is shown by the non-action of a vacuum, of air or gases, and it is also further shown by the special degree in which different matters possess the property. That magnetic force acts upon the ray of light always with the same character of manner, and in the same direction, independent of the different varieties of substance, or their states of solid or liquid, or their specific rotative force, shows that the magnetic force and the light have a direct relation; but that substances are necessary, and that these act in different degrees, shows that the magnetism and the light act on each other through the intervention of the matter. Recognising or perceiving matter only by its powers, and knowing nothing of any imaginary nucleus abstract from the idea of these powers, the phenomena described must strengthen my inclination to trust in the views I have advanced in reference to its nature.”—Phil. Mag. vol. xxiv.
[106] The invention of the camera obscura certainly belongs to Giambattista Porta, and is described in his Magiæ Naturalis, sive de Miraculis Rerum Naturalium, Libri Viginti; Antwerp, 1561. An English translation made in 1658 exists, but I have not seen it.
Hooke, in one of the earliest volumes of the Philosophical Transactions, describes as new many of the phenomena mentioned by Porta, and particularly the images of the dark chamber.
[107] Herschel, On Light,—Encyclopædia Metropolitana.
[108] “I would here observe that a consideration of many such phenomena (the obliteration and revival of photographic drawings) has led me to regard it as not impossible that the retina itself may be photographically impressed by strong light, and that some at least of the phenomena of visual spectra and secondary colours may arise from the sensorial perception of actual changes in progress in the physical state of that organ itself subsequent to the cessation of the direct stimulant.”—On the action of the Rays of the Solar Spectrum on Vegetable Colours, &c.: by Sir J. F. W. Herschel, Bart.
[109] Dumeril.
[110] Theory of Colours: by Goethe; translated by Eastlake.
[111] See Tuckey’s Narrative of the Expedition of the Zaire.
[112] The most complete examination of this subject will be found in two Memoirs:—
1. Experiments and observations on the light which is spontaneously emitted with some degree, of permanency from various bodies.—Phil. Trans., vol. xc.