Fig. 321.
Bargeman throwing his tow-rope into waves to get it over the thick bushes.
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 light is acquired from the inspection of another of Mr. Woodward's cardboard models. (Fig. 323.)
Fig. 322.
A section of a wave of common light made up of the transversal vibration, a b and c d.
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.