at this focus, and the experiment repeated. In this way I have determined the indices of refraction of several solids
Fig. 7.—Electric Refractometer.
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.
