Now if a similar movement is made with the stretched rope from right to left, another wave will be produced, which will run along the cord in an horizontal position, and if the latter is compared with the perpendicular undulation, it will be evident that each set of waves will be in planes at right angles to and independent of each other. This is supposed to be the mechanism of a wave of common light, so that if a section is taken of such an undulation, it will be represented by a circle A B C D (Fig. 322), with two diameters A B, and C D; or a better mechanical notion of a wave of common [Page 337] light is acquired from the inspection of another of Mr. Woodward's cardboard models. (Fig. 323.) [Illustration: Fig. 323. Model of a wave of common light.] The existence of an _alternating motion of some kind_ at minute intervals along a ray is, says Professor Baden Powell, "as real as the motion of translation by which light is propagated through space. _Both_ must essentially be _combined_ in any correct conception we form of light. That this alternating motion must have reference to certain directions _transverse_ to that of the ray is equally established as a consequence of the phenomena; and these _two_ principles must form the basis of any explanation which can be attempted." A beam of common light is therefore to be regarded as a rapid succession of systems of waves in which the vibrations take place in different planes. If the two systems of waves are separated the one from the other, viz., the horizontal from the perpendicular, they each form separately a ray of polarized light, and as Fresnel has remarked, _common light_ is merely _polarized light_, having _two planes_ of polarization at _right angles_ to each other. To follow up the mechanical notion of the nature of polarized light, it is necessary to refer again to Woodward's card wave model (Fig. 323), and by separating the two cards one from the other it may be demonstrated how a wave of common light reduced to its skeleton or primary form is reducible into two waves of polarized light, or how the two cards placed together again in a transversal position form a ray of common light. (Fig. 324.) [Illustration: Fig. 324.--No. 1. Common light, made up of the two waves of polarized light, Nos. 2 and 3. ] The query with respect to the nature of polarized light being answered, it is necessary, in the next place, to consider how the separation of these transversal vibrations may be effected, and in fact to ask what optical arrangements are necessary to procure a beam of polarized light? Light may be polarized in four different ways--viz., by reflection, single refraction, double refraction, and by the tourmaline--viz., by absorption. [Page 338] _Polarization by Reflection, and by Single Refraction._