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nations to their crystalline axes. If the experiment be made with a conical pencil of light, large enough to give the various rays composing its inclinations sensibly different to the axes, so that they experience double refractions sensibly unequal, these rays, analyzed after they emerge, offer different colours united in the same system of polarization; and the union of these colours forms round the axes coloured zones, the configuration of which indicates the system of polarizing action exerted by the substance under consideration. This kind of experiment is therefore very proper to exhibit the axes and to indicate the mode of polarization with which any given substance affects the rays.
Upon the whole, the interferences of polarized rays offer very remarkable properties, many of which have been discovered and analyzed by Messrs. Arago and Fresnel with great ingenuity and considerable success, but as the limits of this Work do not allow of a full exposition of them, I will only cite one, which is, that rays polarized at right angles do not affect each other when they are made to interfere, whereas they preserve that power when they are polarized in the same direction. It is not only crystalline bodies that modify polarization impressed on the rays of light: Messrs. Malus and Biot found by different experiments made about the same time, that if a ray be refracted successively by several glass plates placed parallel to each other, it will at length be polarized in one single direction perpendicular to the plane of refraction. Malus, by a very ingenious analysis of this phænomenon, has moreover shown that it is progressive, the first glass polarizing a small portion of the incident light, the second a part of that which had escaped the action of the first, and so on. M. Arago, measuring the successive intensities by a method of his own invention has shown that they are exactly equal to the quantity of light polarized in contrary directions at each reflexion. A phænomenon analogous to this is produced naturally in prisms of tourmaline, which appear to be composed of a multitude of smaller prisms, united together, but without any immediate contact. All light passing through one of these prisms perpendicularly is found to be polarized in a direction perpendicular to the edges, so that if two such prisms be placed at right angles, on looking through them a dark spot is seen where they cross. This property of the tourmaline affords a very convenient method to impress on a pencil of rays a polarization in
