1897.] 



on the Polarisation of the Electric May. 



299 



circle is mounted on one of the sliding stands. The spectrometer 

 carries a circular platform, on which the various reflectors, refractors, 

 &c., are placed. The platform carries an index, and can rotate inde- 

 pendently of the circle on which it is mounted. The receiver is 

 carried on a radial arm (provided with an index), and points to the 

 centre of the circle. An observing telescope may also be used with 

 a glass objective, and a linear receiver at the focus. 



I shall now exhibit some of the principal experiments on electric 

 radiation. 



T ^ 



Fig. 2. — Arrangement of the Apparatus. One-sixth nat. size. 



R, radiator ; T. tapping key ; S, spectrometer-circle ; M, plane mirror ; 

 C, cylindrical mirror ; p, totally reflecting prism ; P, semi-cylinders ; 

 K, crystal-holder ; F, collecting funnel attached to the spiral spring 

 receiver ; t, tangent screw, by which the receiver is rotated ; V, vol- 

 taic cell ; r, circular rheostat ; G, galvanometer. 



Selective Absorption. 



I arrange the radiation apparatus so that a parallel beam of elec- 

 tric radiation proceeding from the lantern falls on the receiver placed 

 opposite ; the receiver responds energetically, the light-spot from the 

 galvanometer being swept violently across the screen. I now inter- 

 pose various substances to find out which of them allow the radiation 

 to pass through and which do not. A piece of brick, or a block of 

 pitch, is thus seen to be quite transparent, whereas a thick stratum 

 of water is almost opaque. A substance is said to be coloured when 

 it allows light of one kind to pass through, but absorbs light of a 

 different kind. A block of pitch is o^Daque to visible light, but trans- 

 parent to electric radiation ; whereas water, which is transparent to 

 light, is opaque to electric radiation. These substances exhibit selec- 

 tive absorption, and are therefore coloured. 



