46G 
PROFESSOR HENRY A. MIERS : AN ENQUIRY INTO 
purjDOse the eye-piece is provided with an additional cross-wire MM, which can he 
moved across the field of view hy a micrometer screw 8, so that the angular value 
of any displacement along a diameter can he measured in terms of divisions upon 
the head of the drum at S. The movable wire, with its micrometer adjustment, 
is attached to a circular frame, C, which has a milled edge, and can be rotated about 
the centre O through any reipiired angle; this frame is in the form of a case 
enveloping a circular disc which is graduated, and the angle through which it is 
turned is read by means of a vernier, V, engraved upon the side of an aperture cut 
in the case C, througli which the scale is viewed. A toothed scale at L registers the 
number of complete turns which have been imparted to the micrometer screw, each • 
interval corresponding to one turn. In this way the distance OP and the angle POC 
can be accurately measured. R is a screw by which the circle is clamped. 
This eye-piece was subsequently described by Mr. Tutton (‘ Phil. Trans.,’ A, 
vol. 192, 1899, p. 463), who added it to his crystal-grinding apparatus ; it would be 
a useful adjunct to any telescope goniometer.'^ 
For the purpose of measuring crystals immersed in the mother-liquid, one face of 
the glass trough having been set jDerpendicular to the collimator, the telescope is set 
perpendicular to the adjacent face of the trough by the so-called Gauss method, nc., 
by viewing the image of the cross-wires reflected at its surface by means of the light 
derived from a lamp placed beside the telescope and sent down the telescope by a 
glass plate held obliquely in front of tlie eye-piece. 
It was found advisable for the experiments descrilied on pp. 493-515 to fix the trough 
on a table which can be adjusted by means of tiiree screws, so that a side of the 
trough can be accurately set perpendicular to the telescope (see fig. 16, Plate 13). 
The temperature of the solution is recorded by a thermometer immersed in it and 
clamped to the frame of the instrument, as shown in fig. 1, Plate 13. 
(4.) Purposes for which the Goniometer can be used. 
The goniometer here described can clearly be used for any work tvhich the ordinary 
goniometer is required to perform. In can further serve several other purposes. 
Grystals can be measured, as in tlie present research, while immersed in a concen¬ 
trated solution of their own material, and the changes of form which they experience 
during growth may thus be studied ; deliquescent crystals, or such as are liable to 
alteration under ordinary conditions, can be measured while immersed in oil or in 
some other appropriate liquid; the action of solvents on crystals can be studied by 
measuring them while immersed in acids or other solvents, and this method aftords 
the only safe means of ascertaining the angles made by the prerosion laces due to the 
* Another telescopic adjunct, described in the same memoir by IMr. Tutton, as devised by mj’self 
(p. 4G4), namely, the swinging lens before the eye-piece, was not, as might appear from his description, 
used with this instrument, but with another goniometer. 
