o48 Professor Oliver Lodyc [June 1, 



but every possible kind of wave must give iuterfereuce and diffraction 

 effects, and their theory is, so to say, worked out. More interest 

 attaches to polarisation, double refraction and dispersion experi- 

 ments. 



Polarisation experiments are easy enough. Radiation from a 

 sphere is already strongly polarised, and the tube acts as a partial 

 analyser, responding much more vigorously when its length is 

 parallel to the line of sparks than when they are crossed ; but a 

 convenient extra polariser is a grid of wires something like what 

 was used by Hertz, only on a much smaller scale ; say an 18-in. 

 octagonal frame of copper strip with a harp of parallel copper wires 

 (see Fig. 21, on floor). The sjmrk-lme of the radiator being set at 

 45°, a vertical grid placed over receiver reduces the deflection to 

 about one-half, and a crossed grid over the source reduces it to nearly 

 nothing. 



Rotating either grid a little, rapidly increases the effect, which 

 becomes a maximum when they are parallel. The interposition of 

 a third grid, with its wires at 45° between two crossed grids, restores 

 some of the obliterated effect. 



Radiation reflected from a grid is strongly polarised, of course, in 

 a plane normal to that of the radiation which gets through it. They 

 are thus analogous in their effect to Nicols, or to a pile of plates. 



The electric vibrations which get through these grids are at right 

 angles to the wires. Vibrations parallel to the wires are reflected or 

 absorbed. 



To demonstrate that the so-called plane of polarisation of the 

 radiation transmitted by a grid is at right angles to the electric 

 vibration,* 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 i^arafifin 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 j)erpendicular 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. 



* Cf. Trouton, in ' Nature,' vol. xxxix. p. 393 ; and many optical experiments 

 by Mr. Trouton, vol. xl. p. 398. Also by Klemencik, (Wied. Ann. vol. xlv.). 

 Righi (Ace. dei Lincei, vol. xi.), and Elster and Geitcl (Phil. Mag. July 1894, 

 p. 158). 



