Page:Philosophical magazine 23 series 4.djvu/115

 of refraction of that ray in the body,

Also $$v$$, the velocity of light in the substance, is related to V, the velocity of light in air, by the equation

Hence if $$z$$ be the thickness of the substance through which the ray passes, the angle through which the plane of polarization will be turned will be in degrees,

or, by what we have now calculated,

In this expression all the quantities are known by experiment except $$r$$, the radius of the vortices in the body, and $$s$$, the density of the luminiferous medium in air.

The experiments of M, Verdet supply all that is wanted except the determination of Z in absolute measures and this would also be known for all his experiments, if the value of the galvanometer deflection for a semirotation of the testing bobbin in a known magnetic field, such as that due to terrestrial magnetism at Paris, were once for all determined. 

XV. On the Composition, Structure, and Formation of Beekite. By, B. A. Oxon, F.C.S.

[With a Plate.]

THERE occurs in the triassic red conglomerate of Torbay and its neighbourhood, an interesting siliceous substance (generally considered to be a variety of hornstone), which offers a problem not only to the geologist and palaeontologist, but also to the chemist. The Beekite is, in fact, not a mineral merely, but a fossil which has been more or less completely mineralised, the mineralization having, however, been effected in a way not very easy to understand. In the present paper, after having quoted some authorities in order to show the geological character and position of Beekites, I shall endeavour to throw some light, 