1 10 Instrument for grinding Section-plates of Crystals. [Feb. 1 , 



principal directions of optical elasticity is known from goniometrical, 

 stanroscopical, and convergent light observations, is first adjusted 

 parallel to the axis and with one or two pairs of faces parallel to one 

 or both of the planes of circular motion, and subsequently by move- 

 ment of one or both of the segments over the calculated arc or arcs, 

 the desired direction can be adjusted accurately perpendicular to the 

 grinding disc. Provision for setting any face exactly parallel to 

 a circular motion is provided in a special crystal holder which per- 

 mits of the requisite amount of rotation after the holder is fixed in its 

 socket. The grinding disc is lubricated with a mobile oil incapable 

 of attacking the crystal, so that the reference faces may be preserved 

 for checking the adjustment after grinding. The crystal is brought 

 down near to the grinding disc by lowering the inner axis, and it is 

 then lowered into contact with the disc and the pressure with which 

 it bears during the grinding regulated by manipulation of the levers. 



For grinding the parallel surface the crystal, mounted by its 

 ground surface upon a very small glass disc, is enclosed in a recep- 

 tacle for it in a special crystal holder consisting of two parts screwed 

 together, the crystal downwards so as to pass through a central 

 aperture in the lower portion in which the disc is supported by a 

 very thin annnlns. The broad under surface of the upper portion of 

 the holder, against which the glass disc is pressed when the two 

 parts are screwed together, is made truly plane, and can be adjusted 

 accurately parallel to the grinding disc. The disc and crystal 

 may be removed from time to time during the grinding without 

 disturbing the adjustment, so that grinding may be stopped when 

 the section exhibits small rings in its interference figure. Half an 

 hour is ample for grinding and polishing the two surfaces, and it is 

 unusual to break a crystal, provided the left hand is used to control 

 the nearest lever, and thus ascertain by the delicate sense of touch 

 how the crystal is supporting the strain, when the lever may be 

 manipulated, almost involuntarily, accordingly. 



Prisms are quite as readily ground and polished. It is possible 

 in most cases to grind two surfaces symmetrically inclined to a plane 

 of optical elasticity and with the edge of intersection parallel to an 

 axis of optical elasticity, by suitable adjustment of the crystal with 

 the aid of the movements provided and with reference to existing 

 faces. 



