Page:Proceedings of the Royal Society of London Vol 60.djvu/16

8 necessary to first grind plates of known orientation for optical examination; this latter operation is by no means easily performed, especially in the case of brittle organic substances. Very usually, however, crystals belonging to the biaxial systems are obtained in which an optic axis apparently emerges into air through a particular face ; in these cases the accurate measurement of the angle between the apparent direction in air of the optic axis and the normal to the crystal plate becomes an important element in the determination of the optical constants of the crystal.

The ordinary method of determining this angle is a direct one ; the crystal is adjusted in the optic axial angle apparatus and a reading is taken for the above emergence, after the position of the normal to the plate has been found by reflecting a beam of light down the telescope tube and turning the crystal until the shadow and reflected image of the crosswires coincide; the angular difference between the two readings is then the required apparent angle of emergence into air. This method of finding the position of the normal is, however, very tedious, and, unless the crystal plate possesses a highly polished surface, very inaccurate.

To remedy these defects a method has been devised of indirectly determining this angle by calculating it from the angle through which the optic axis is apparently refracted by an oil of high refractive index. The crystal is mounted and adjusted in the optic axial angle apparatus in the ordinary way, and a reading is taken for the optic axial emergence in air; a parallel-sided glass cell containing a-bromonaphthalene or some other highly refractive liquid is then raised until it surrounds the crystal, and a second reading is taken of the apparent emergence of the optic axis. From the difference between these two angular readings the angle of emergence into air can be calculated, if the index of refraction of the oil is known.