4G4 MR. A. E. TUTTON ON THE THERMAL DEFORMATION OF THE CRYSTALLISED 
being diameters of the circular field ; and also a third spider-line which is both 
rotatable and capable of movement j^erpendicular to itself in the focal plane. The 
fixed lines are attached in the central aperture of a circle-plate 7 centims. diameter 
fixed round the optical tube of the eyepiece, and which carries near its periphery a 
circle divided directly into degrees. The movable spider-line is carried in the aper¬ 
ture of a micrometer box carried in front of a similar circular plate, which latter is 
fitted closely to the former plate in front of it, and partly enveloping it Avith a milled 
flange in such a manner as to be rotatable about it. This front plate is pierced by 
a window above the micrometer box in such a position that the divided circle of the 
fixed jolate is visible through it. The inner edge of the windoAv is bevelled and carries 
a vernier, with the aid of which the circle reads to minutes. The movable line is 
fixed to the front of the traversing frame of the micrometer, at the focus of the double 
eyepiece which slides in the short portion of the optical tube in front of the box. The 
fixed lines are brought into the focal plane and almost into contact Avith the moAmble 
one by means of a relatively thick anmdus capable of penetrating the traAmrsing frame 
and attached to the aperture of the fixed circle. The traAmrse of the frame and its 
spider line is recorded by a divided drum of the usual kind at the right-hand side of 
the box; the drum is divided into 100 parts, and the reading is indicated by an index 
mark fixed alongside. Tlie moAmhle circle and the spider line AAdiich it carries can be 
clamped to the fixed circle and the stationary spider lines Avhen desired, by means of 
a suitable clamping screAv proAuded Avith milled head, on the lower part of the peri¬ 
phery o])posite to that near Avhich the AvindoAAA is situated. These arrangements 
enable small movements of the image of the collimator signal, reflected from a crystal 
surface during groAAdh and due to disturbance of the thermal or other conditions of 
the solution, to l^e folloAved and measured, Avhether they are lateral, angular, or both. 
The other addition to tlie telescope is that of so arranging the remoA'able lens, 
usually added to the telescopes of goniometers outside the ohjectiA^e for the jAurpose 
of conA^ertlng the optical system into that of a Ioav poAver microscope focussing the 
crystal, as to make it capable of being throAvn into position either as usual behind the 
objective or in front of the eyepiece, and further- of making it capable of traA^elling 
for some distance along the o|)tical axis. The jnirpose of this is to enable the image 
of the signal to be actually folloAved right up to the image of the crystal itself, in order 
to he quite certain as to the particular face from Avhich it emanates. This is achieA'ed 
]3y supplying tAvo such lenses, mounting the pair on a T-piece, and hinging the stem 
ahont a small j^latform carried ahoAm a short tube sliding round the main optical tube 
and prevented from rotation by a suitable rib and grooA^e. It is only necessary to 
SAving the T-plece over one AAmy or the other for the lens to fall into joosition either 
adjoining the ohjectiAm or the eyepiece, the length of the end cross-piece carrying the 
lenses being arranged so that either lens falls exactly into the optical axis. The one 
AAdiich falls behind the ohjectiA'e is generally employed close \ip to the latter in the 
usual manner. The other one AAdiicli falls in front of the eyejiiece is the one employed 
