PLATES AND PRISMS OF CRYSTALS OP ARTIFICIAL PREPARATIONS. 901 
distortion is better still, as being probably within one or two minutes of 90°. "the 
object chosen is placed near the edge of the grinding disc, resting upon one of the 
true surfaces. It is so arranged that the other surface, inclined at 90°, reflects the 
image of the curved slit of the collimator, illuminated by the lamp, along the axis of 
the telescope. It is then observed whether the image of the slit is bisected by the 
horizontal cross-wire, as it ought to be if the grinding plane is parallel to the plane of 
the optical tubes. If this is the case, it only proves that the particular diameter of 
the grinding plane which is parallel to the normal to the reflecting face is correctly 
adjusted ; the plane may still be tilted about this diametei'. A second observation, 
with the reflecting object rotated on the flxed plane for about a right angle, is 
necessary to ascertain this. The telescope and collimator are therefore moved round, 
each about 90° upon their guiding arcs, the lamp is also correspondingly moved, and 
the reflecting object is likewise moved round until the image of the slit is again 
observed in the centre of the field. If in both positions the image is symmetrical to 
the horizontal cross-wire, the parallelism of the grinding surface and the plane of the 
optical tubes is established. The test is still more delicate if the slit is arranged 
horizontally instead of vertically. If in either or both of the positions the image is 
not symmetrical to the horizontal spider-line, the levelling screws of the grinding 
table must be adjusted, by means of a tapering steel rod supplied, slightly bent near 
one end so as to permit it to be inserted more conveniently into the holes of the screw 
heads, until such is the case. 
The Grinding of the First Surface of a Section-plate. 
In describing the mode of grinding the first surface of a section-plate, it will be 
convenient to consider four typical cases, taken from biaxial crystals, which will 
illustrate the uses of the various movements provided with the instrument. 
1. The simplest case is that of a crystal belonging to the rhombic system which 
exhibits a well-marked zone comprising two pairs of pinacoid faces, or a pair of 
pinacoid faces and faces of the basal plane, together, perhaps, with interlying prism or 
dome faces, or consisting of prism or dome faces alone. Let the axis of this zone be 
the median line to which it is desired to grind a section perpendicular, such a section 
not being available ready formed owing to the absence or inadequate development of 
faces (pinacoidal or basal) parallel to the plane in question. The crystal is cemented 
upon the holder by means of the easily fusible but rapidly setting wax, previously 
referred to ; it should be well embedded in the wax, which should also be pressed 
closely round it and into the grooves of the holder while warm, attention to these 
points being essential in order to avoid fracture during grinding. The crystal is 
arranged with the zone of faces referred to parallel to the axis of the holder, so that 
when the latter is fixed in its socket the zone is approximately vertical. The 
telescope is then fixed in a convenient position front,ing the observer, the collimator at 
