464 
rROFERSOK HENRY A. I^IIERS : AN ENQ.UIRY INTO 
Ill measuring crystals in the ordinary way it is very difficult to know whether the 
faces are plane and smooth until tlie crystal has been taken from the solution and 
dried, and then one cannot feel sure that their surface has not been altered by the 
evaporating solution during the process. By measuring crystals while still immersed 
in the solution this difficulty also would be oliviated. I feared at first that when 
immersed in a medium of so liigh a refractive index as the saturated solution, 
the faces of a growing crystal would not yield sufficiently brilliant images of the 
collimator signal. This, however, is not the case ; and having convinced myself hy 
preliminary experiments that very good images can l)e ol)tainecl with the ordinary 
telescope and collimator, I had an instrument constructed which is really nothing 
more than a telescope-goniometer, inverted so that the crystal is suspended in the 
solution. Another advantage of this method is that the zone which is being- 
measured has all its faces vertical, so that the possible distorting action of gravity is 
eliminated. The li(.[uid is contained in a glass vessel with plane-parallel sides, 
through one of which the light enters normally from the collimator, while through 
another the reflected light emerges normally to enter the telescope. 
A grant for the j^^n’pose having been obtained from the Tloyal Society in 1892, the 
goniometer was constructed according to my design by Messrs. Teoughton and 
Simms. 
(3.) The Goniometer. 
The graduated circle of the goniometer, fig. 1 (Plate 13), rests upon a horizontal 
disc which is carried by two vertical metal columns about 20 centims. in height ; 
tliese spring from the base plate of the instrument ; and in an aperture in one of 
them is fixed the collimator. The vernier is engraved upon the rim of the fixed di.sc. 
The telescope is fitted to the lower end of a solid counterpoised arm, which can be 
rotated about tlie goniometer axis and clamped in any desired position. The crystal 
holder, which may consist of tlie usual centring and adjusting movements, is at the 
lower end of a vertical axle which passes through the centre of tlie goniometer disc, 
and may be raised or lowered by a screw with milled head, so as to bring the crystal 
into the horizontal plane of the telescope and collimator.'^ The axle of the crystal 
holder is clamped hy a split ring grip above the bolder. The glass trough which 
contains the solution is a square vessel 6 centims. square, with plane-paj’allel sides 
mounted in a brass frame ; it stands upon a lirass table, wliich can be raised or 
lowered by rack and pinion fitted below the base-plate, so as to firing the crystal into 
the solution or out of it. A brass guide upon the table receives one side of the frame, 
* For economy of space, the centring movements in this instrument were not effecterl hy the usual 
slides travelling between straight guides, but two plane discs, which swing about pivots and move in 
contaet with one another, as described for another instrument in ‘ Mineralogical Magazine,’ vol. 10, 1SS7, 
p. 215, 
