Spectra more beautiful and intense than the prismatic image,—systems of rings far excelling those of thin plates,—and forms of the most symmetric order, are constantly presenting themselves, as the polarized ray is passed through various transparent substances; the path of the ray indicating whether the crystal has been formed round a single nucleus or axis, or whether it has been produced by aggregation around two axes. The coloured rings, and the dark or luminous crosses which distinguish the path of the polarized ray, are respectively due to different states of tension amongst the particles, although those differences are so slight, that no other means is of sufficient delicacy to detect the variation.
The poetry which surrounds these, in every way, mysterious conditions of the solar beam, is such, that it is with difficulty that imagination is restrained by the stern features of truth. The uses of this peculiar property in great natural phenomena are not yet made known to us; but, since we find on every side of us the natural conditions for thus separating the beam of light, and effecting its polarization, there must certainly be some most important end for which it is designed by Him who said, “Let there be Light.”
It must not be forgotten that we have at command the means of showing that the chromatic phenomena of polarized light are due to atomic arrangement. By altering the molecular arrangement of transparent bodies, either by heat or by mere mechanical pressure, the unequal tension or strain of the particles is at once indicated by means of the polarized ray of light and its rings of colour. Differences in the chemical constitution of bodies, too slight to be discovered by any other mode of analysis, can be most readily and certainly detected by this luminous investigator of the molecular forces.[101]
Although we cannot enter into an examination of all the conditions involved in the polarization of, and the action of matter on, ordinary light, it will be readily conceived, from what has been already stated, that some most important properties are indicated, beyond those which science has made known.
Almost every substance in nature, in some definite position, appears to have the power of producing this change upon the solar ray, as may be satisfactorily shown by examining them with a polarizing apparatus.[102] The sky at all times furnishes polarized light, which is most intense where it is blue and unclouded, and the point of maximum polarization is varied according to the relative position of the sun and the observer. A knowledge of this fact has led to the construction of a “Solar Clock,”[103] with which the hour can be readily determined by examining the polarized condition of the sky. It has been stated, that chemical change on the Daguerreotype plates and on photographic papers is more readily produced by the polarized than by the ordinary sunbeam.[104] If this fact be established by future investigations, we advance a step towards the discovery so much desiderated of the part it plays in natural operations.
The refined and accurate investigations of Dr. Faraday stand prominently forward amid those which will redeem the present age from the charge of being superficial, and they will, through all time, be referred to as illustrious examples of the influence of a love of truth for truth’s sake, in entire independence of the marketable value, which it has been unfortunately too much the fashion to regard. The searching examination made by this “interpreter of nature” into the phenomena of electricity in all its forms, has led him onward to trace what connexion, if any, existed between this great natural agent and the luminous principle.
By employing that subtile analyzer, a polarized ray, Dr. Faraday has been enabled to detect and exhibit effects of a most startling character. He has proved magnetism to have the power of influencing a ray of light in its passage through transparent bodies. A polarized ray is passed through a piece of glass or a crystal, or along the length of a tube filled with some transparent fluid, and the line of its path carefully observed; if, when this is done, the solid or fluid body is brought under powerful magnetic influence, such as we have at command by making a very energetic voltaic current circulate around a bar of soft iron, it will be found that the polarized light is disturbed; that, indeed, it does not permeate the medium along the same line.[105] This effect is most strikingly shown in bodies of the greatest density, and diminished in fluids, the particles of which are easily moveable over each other, and has not hitherto been observed in any gaseous medium. The question, therefore, arises,—does magnetism act directly upon the ray of light, or only indirectly, by producing a molecular change in the body through which the ray is passing? This question, so important in its bearings upon the connexion between the great physical powers, will, no doubt, before long receive a satisfactory reply. A medium is necessary to the production of the result, and, as the density of the medium increases, the effect is enlarged: it would therefore appear to be due to a disturbance by magnetic force of the particles which constitute the medium employed.
Without any desire to generalize too hastily, we cannot but express a feeling,—amounting to a certainty in our own mind,—that those manifestations of luminous power, connected with the phenomena of terrestrial magnetism, which are so evident in all the circumstances attendant upon the exhibition of Aurora Borealis, and those luminous clouds which are often seen, independent of the Northern Lights, that a very intimate, relation exists between the solar radiations and that power which so strangely gives polarity to this globe of ours.
In connexion with the mysterious subject of solar light, it is important that we should occupy a brief space in these pages with the phenomena of vision, which is so directly dependent upon luminous radiation.
The human eye has been rightly called the “masterpiece of divine mechanism;” its structure is complicated, yet all the adjustments of its parts are as simple as they are perfect. The eye-ball consists of four coats. The cornea is the transparent coat in front of the globe; it is the first optical surface, and this is attached to the sclerotic membrane, filling up the circular aperture in the white of the eye; the choroid coat is a very delicate membrane, lining the sclerotic, and covered with a perfectly black pigment on the inside; and close to this lies the most delicately reticulated membrane, the retina, which is, indeed, an extension of the optic nerve. These coats enclose three humours,—the aqueous, the vitreous, and the crystalline humours.