the Study of tlie Properties of Electric Waves. 



65 



at this focus, and the experiment repeated. In this way I 

 have determined the indices of refraction of several solids 



Fig. 7. — Electric Kef lactometer. 



R, the Radiator. C, the Receiver. 



for the electric ray (vide " On the Determination of the 

 Indices of Refraction of various Substances for the Electric 

 Ray," Proc. Roy. Soc. vol. lix.). The index of refraction 

 of commercial sulphur is= 1*73 ; that of a specimen of pitch 

 = 1-48. 



Indices for Liquids. — A cylindrical trough is filled with the 

 given liquid ; two thin parallel glass plates enclosing an air- 

 space are vertically placed so as to divide the liquid cylinder 

 into two halves. The readings for total reflexion are taken 

 as in the last case. The index for a specimen of coal-tar I 

 found to be 1'32. 



Selective Absorption. 



A substance is said to be coloured when it allows light of 

 one kind to pass through, but absorbs light of a different 

 kind. If we take into account the entire range of radiation 

 there is hardly a substance which is not, in this sense, 

 coloured. In the spectrum of radiation transmitted through 

 glass, for example, two broad absorption-bands would be 

 observed, one in the ultra-violet, and the other in the infra- 

 red, the electric and the visible rays not being absorbed to 

 any great extent. A brick or a block of pitch would absorb 

 light, but would transmit the electric ray. On the other 

 hand, a stratum of water, though transparent to light, would 

 absorb the electric ray. These substances exhibit selective 

 absorption, and are therefore coloured. 



Phil. Mag. S. 5. Yol. 43. No. 260. Jan. 1897. F 



