8 Mr. W. J. Pope. 



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 read- 

 ing 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 differ- 

 ence 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 pos- 

 sesses 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 refrac- 

 tive 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. 



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