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.