Page:The Annual Register 1899.djvu/530

 106 SCIENCE. [1899.

beam of light from an arc-lamp can pass through them parallel to the lines of force. The beam of light is received by an analyser which is turned to extinction before the magnet is excited ; and when the current is transmitted, brightness is restored.

It is supposed that when light of frequency N is passed along the lines of a magnetic field, it is split up into two sets of circular waves, a right-handed and accelerated, and a left-handed and retarded set ; so that two frequencies are produced N + n and N — n. But N, which the analyser was adjusted to extinguish, no longer exists, having been transformed, by magnetic force, into N + n and N — n, and these un- arrested became visible.

If the absorbing substance between the pole-pieces be nitric-oxide fumes which absorb green, the observed light is red until the magnetic field is established, and then it becomes blue-green.

Professor Righi has determined the rotatory efficiency of chlorine in a magnetic field to be 0000337, which may be compared with the values 0-000302 and 0000393 found by M. Becquerel for carbonic anhydride and protoxide of nitrogen respectively.

Professor Fitzgerald has suggested that circularly polarised light sent through an absorbing medium ought to constitute a magnet. A radiating atom in a magnetic field gives out circularly polarised light. A circularly polarised beam of light should in its turn cause a directed rotation of the electrons, so that the absorbing gas should be magnet- ised and exhibit magnetic force. Professor Righi has investigated the problem both with the vapour of hypoazotide and with that of bromine, using an astatic magnetometer which was sensitive to a magnetic field of 10-* C.G.S. units; but no perceptible effect was produced.

The quartet form of the Zeeman spectral lines, which consists of two strong side lines, with two fainter lines between them, has no real existence as a type, since the side lines themselves separate into pairs when the magnetic field is strengthened to 50,000 C.G.S. units. In Professor Preston's opinion these and similar effects are produced by the action of the magnetic field upon the vibrating structure which excites the radiation. Lord Kelvin follows M. Lorentz in accounting for the facts by an assumption concerning the ions. These are no longer to be regarded as simple electrified particles, but as complex dynamical systems. Carrying separated charges of both signs of electricity, they experience, in the magnetic field, equal and opposite tangential forces. They are consequently set in rotation, and give rise, by kinetic energy, to ethereal waves.

Another" vacuum effect has been observed by Mr. Phillips. On stopping an electric discharge which has been passing between soft iron electrodes in a Crookes* tube, and then setting up a magnetic field between them, a luminous ring forms, with its plane at right angles to the lines of force and in rotation about the magnetic axis. The direction of rotation is that which would be communicated to negatively charged particles and is reversed on reversing the magnetic field. Other vacuum-tube phenomena, of a kindred sort, have been observed by M. Fomm and by Sir W. Crookes.

Uranium, condemned by periodic schemes to a hopeless inaction,

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