PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS 895 
easily estimated. When any of the indicating marks are brought opposite the zero 
of the circle the movements are either parallel or crossed at 90°, so that when the 
movements are otherwise inclined the inclination is immediately given by the circle 
reading- indicated. The lower movement may be clamped to the upper one in any 
position by means of a double tightening screw, which fixes the indicating disc to the 
circle, and which can be manipulated from eitlier side by means of a key supplied. 
The crystal is directly attached, by means of a readily fusible but quickly and 
rigidly setting wax, to a small holder consisting of a brass disc deeply cross-grooved 
on the under side in 07’der to prevent the slipping of the wax ; to the centre of the 
disc a short steel rod is attached, w'hich slides easily in a hole bored in the centre of 
the under surface of the lowest segment of the adjusting apparatus, rotation being 
again prevented by grooving the rod and fixing a corresponding rib in the hole. 
Two such holders are provided, one whose disc has a diameter of f inch and another 
of f inch, in order to suit smaller and larger crystals. The smaller one is seen in 
position in fig. 1, and the larger one lies on the circular base-board somewhat to the 
left. In addition to these, two special holders are provided, one of which permits of 
a certain amount of rotation of the crystal, in order to be able to adjust any face 
