1897.] on the Polarisation of the Electric Bay, 303 



principal plane of the crystal coincides with the polarisation planes of 

 either the polariser or the analyser. The field also remains dark when 

 the optic axis of the crystal is parallel to the incident ray. 



A similar method is adopted for experimenting with polarised 

 electric radiation. 



The spectrometer-circle is removed from the optical bench, and 

 an ordinary stand for mounting the receiver substituted. By fitting 

 the lens-tube, the electric beam is made parallel. At the end of the 

 tube may be fixed either the grating polariser or the jute or serpen- 

 tine polarisers, to be subsequently described. 



The receiver fitted with the analyser is adjusted by a tangent 

 screw, the rotation of the analyser being measured by means of an 

 index and a graduated vertical disc. 



The polarising gratings may be made, according to Hertz, by 

 winding copper wires, parallel, round square frames. The polari- 

 sation apparatus is, however, so extremely delicate, that unless 

 all the wires are strictly parallel, and the gratings exactly crossed, 

 there is always a resolved component of radiation which acts on the 

 sensitive receiver. It is a very difficult and tedious operation to 

 cross the gratings. I have found it to be a better plan to take two 

 thick square plates of copper of the same size, and, placing one over 

 the other, cut a series of slits (which stop short of the edges) parallel 

 to one of the edges. One of these square pieces serves as a polariser, 

 and the other as an analyser. When the two square pieces are ad- 

 justed, face to face, with coincident edges, the gratings must either be 

 parallel or exactly crossed. Such accurate adjustments make it pos- 

 sible to carry out some of the most delicate experiments. 



The radiator-tube, with the lens and the attached polariser, is 

 capable of rotation. The emergent beam may thus be polarised in a 

 vertical or a horizontal plane. The analyser fitted on to the receiver 

 may also be rotated. The gratings may thus be adjusted in two 

 positions. 



(1) Parallel position. 



(2) Crossed position. 



In the first position the radiation is transmitted through both the 

 gratings, falls on the sensitive surface, and the galvanometer responds. 

 The field is then said to be bright. In the second position the radia- 

 tion is extinguished by the crossed gratings, the galvanometer re- 

 mains unaffected, and the field is said to be dark. But in interposing 

 a double-refracting substance in certain positions between the crossed 

 gratings, the field is partially restored, and the galvanometer-spot 

 sweeps across the scale. 



I have now the analyser and the polariser exactly crossed, and 

 there is not the slightest action on the receiver. Observe the great 

 sensitiveness of the arrangement ; I turn the polariser very slightly 

 from the crossed position, and the galvanometer-spot is violently 

 deflected. 



