PLATES AND PRISMS OF CRYSTALS OF ARTIFICIAL PREPARATIONS. 899 
surface on the side upon which the handle is not placed is made a true plane. A 
disc of the finest emery cloth is cemented to it by a thin film of any suitable liquid 
cement. The surface of the grinding disc is moistened with turpentine, and the 
little emery plane is moved to and fro diametrically over it ; the surface thus ground 
bites better in grinding a crystal and does not produce striae upon the crystal surface 
so much as when the grinding is done concentrically. 
The instrument is conveniently mounted upon a rigid rectangular box, which is 
best not quite so broad as the mahogany base-board, in order that the telescope may 
be at the height of the observer’s eye when seated, and that the eye may he 
conveniently approached quite close to the eyepiece. 
As the instrument is usually employed in a darkened room or at night a lamp is 
required. The table lamp, fitted with the most recent rare-earth mantle and burner, 
and with an opal shade, supplied by the Incandescent Gas Light Company, is 
admirably adapted, especially when it is arranged to be able to lower the flame till it 
is all but extinguished, and to instantly raise it again as often as may be desired by 
means of a lever-tap fitted with stop-pin, and fixed within reach under the table. 
An electric incandescent lamp manipulated by’’ a table switch is equally suitable. 
In addition to the above table lamp a goniometer lamp is required. One which 
has been specially constructed to meet the requirements of goniometrica,! work is 
employed by the author. It is shown in the background in fig. 7 of the communica¬ 
tion concerning the new monochromatic light apparatus (p. 933). A mantle and burner 
with glass or mica chimney, similar to that of the table lamp, but fitted in addition 
with by-pass, are supported upon an arm capable of sliding upon a tall standard and of 
being fixed at any height by means of a clamping screw. The observer is shielded 
from the brilliant light by means of an enveloping copper cylinder supported in a 
ring, whose arm is likewise capable of sliding along and of being clamped to the 
standard, a counterpoise being provided to facilitate the sliding. A circular aperture, 
inch in diameter, is cut in the cylinder at a little more than one-third of its 
height. The slider which supports the cylinder is first adjusted so that the aperture 
is opposite the end of the collimator, and the slider which supports the lamp is then 
adjusted so that the brightest part of the mantle is opposite the centre of the 
aperture and the slit of the collimator, and in a continuation of the axial line of the 
latter. During the goniometrical operation of bringing an image of the slit, reflected 
from a crystal face, to the cross-wire of the telescope, the table light is switched off 
and the observer is shaded from stray light from the aperture of the copper cylinder 
by means of a screen enveloping the half of the instrument nearest the lamp, and 
which is pierced by a hole for the passage of the collimator. In order to read the 
vernier the table light is temporarily switched on; it is again turned down while 
bringing the next image to the cross-wire, and so on, the operations of switching on 
and ofi* being readily performed with one hand, while using the other in manipulating 
the instrument and recording the readings. When the goniometrical observations 
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