_Reflecting Paraffin Surface; Direction of Vibrations in Polarised Light._ To demonstrate that the so-called plane of polarisation of the radiation transmitted by a grid is at right angles to the electric vibration,[17] _i.e._, that when light is reflected from the boundary of a transparent substance at the polarising angle the electric vibrations of the reflected beam are perpendicular to the plane of reflection, I use the same paraffin prism as before; but this time I use its largest face as a reflector, and set it at something near the polarising angle. When the line of wires of the grid over the mouth of the emitter is parallel to the plane of incidence, in which case the electric vibrations are perpendicular to the plane of incidence, plenty of radiation is reflected by the paraffin face. Turning the grid so that the electric vibrations are in the plane of incidence, we find that the paraffin surface set at the proper angle is able to reflect hardly anything. In other words, the vibrations contemplated by Fresnel are the electric vibrations; those dealt with by McCullagh are the magnetic ones. Thus are some of the surmises of genius verified and made obvious to the wayfaring man. END OF LECTURE. [17] _Cf._ Trouton, in _Nature_, Vol. 39, p. 393; and many optical experiments by Mr. Trouton, Vol. 40, p. 398. Since then the above described and depicted apparatus for electro-optic experiments has been imitated in a neat, compact form by Prof. J. Chunder Bose, of Calcutta, and with it he has obtained many admirable and interesting optical results. See _Proc._ Roy. Soc. NOTE WITH REFERENCE TO ELECTRIC WAVES ON WIRES. It may be well to explain that in my Royal Institution lecture I made no reference to the transmission of waves along _wires_. I regard the transmission of waves in _free space_ as the special discovery of Hertz; though undoubtedly he got them on wires too. Their transmission along wires is, however, a much older thing. Von Bezold saw them in 1870, and I myself got quantitative evidence of nodes and loops in wires when working with Mr. Chattock in the session 1887-8 (_see_, for instance, contemporary reports of the Bath Meeting of the British Association, 1888, in _The Electrician_), and I exhibited them some time afterwards to the Physical Society, the wires themselves becoming momentarily luminous at every discharge except at the nodes, thus enabling the waves to be actually seen, having been made stationary by reflexion as in the corresponding acoustic experiment of Melde. This experiment does not appear to have been properly known (p. 78). [Illustration: FIG. 23.]