disturbance was eliminated by making the inner box of soft iron which acted as a magnetic screen.
The Receiver.—A satisfactory form of receiver was constructed of steel or of nickel; a single point contact receiver made of steel, nickel, aluminium or magnesium is also found to be highly sensitive and reliable. The sensitiveness can be exalted to any degree desirable by adjustment of electromotive force acting on the circuit. Angular measurements are made with the spectrometer circle (p. 87).
2. Selective absorption
The transparency or opacity of various substances, brick, wood, pitch and others as well as strata of different liquids and solutions can easily be demonstrated by interposing them on the path of electric beam. Water is opaque on account of absorption of radiation; liquid air is, on the other hand, quite transparent. Sheets of metal do not transmit, but reflect the beam according to laws of reflection. A long trough filled with irregularly shaped pieces of pitch is opaque on account of multiplicity of reflection and refraction. It becomes more or less transparent when partial homogeneity is restored by pouring in kerosene (p. 89).
3. Determination of Index of Refraction
The prism method is quite unsuitable for exact determination of the index for the electric ray. Accurate results have, however, been obtained by the method of total reflection. The rotation of two semi-cylinders separated by parallel air-space, produce a sudden extinction of the image at the critical angle. The index of refraction of glass for the electric ray is found to be
