Societies and Academies | Nature

ABSTRACT LONDON. Mineralogical Society, March 21.—W. Barlow, president, in the chair.-Dr. J. W. Evans: A new microscope accessory for use in the determination of the refractive indices of

minerals. The accessory-a diaphragm with narrow slit adjustable in width-when placed in the primary focus of the objective or any point conjugate with it, serves several useful purposes. If

placed parallel to the boundary between the two substances the refractive indices of which are to be compared by the Becke method, it gives better results than an iris diaphragm. In the case

of doubly-refractive sections or grains in which an axis of optical symmetry lies at right angles to the microscope axis, the slit is placed parallel to the former axis, so that the paths

of all the rays of light traversing it lie in a plane of optical symmetry and one direction of vibration is always parallel TO the axis of optical symmetry, and a nicol is inserted so that

the direction of vibration of the rays traversing- it is parallel to the same axis; then the refractive indices of light vibrating parallel to that axis of optical symmetry may be

investigated by the usual methods without the confusion caused by the bifocal images described by Sorby. -L. J. Spencer: A butterfly twin of gypsum. In a well-developed twin-crystal, 6 in.

across, from Gir-genti, Sicily, in which the twin-plane is d(101), the two individuals are situated on the same side of the twin-plane instead of on opposite sides as in the usual type.-Dr.

W. R. Jones: The alteration of tourmaline. In a moist, tropical climate minerals which are ordinarily regarded as stable break down to an extraordinary degree. At Gunong Bakau, Federated

Malay States, tourmaline is found more or less completely altered to a mica (probably phlogopite) and limonite, the degree of alteration decreasing with increasing- depth from the surface,

suggesting that the change was caused by the percolation of water from above. The freshness of tourmaline grains in sands is very probab'y due to the removal of the altered products by

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