894 
MR. A. E. TUTTOX ON AN INSTRUMENT FOR GRINDING SECTION- 
manner, by means of a milled head x, but in a direction at right angles to that 
brought about by the movement of the upper screw. 
The adjusting movements y' and z are the usual pair of circular motions employed 
on the most accurate goniometers, bi'ought about by endless screws and segments of 
wheels, arranged at right angles to each otlier. They are constructed more strongly 
than usual, and particular care has been taken that the axes of the two movements 
are as nearly as pos.sible crossed at 90°. An innovation is introduced, however, 
in order to be able to adjust the crystal so that any desired direction in it may 
be exactly parallel to the grinding plane. This consists in graduating the move¬ 
ments. Upon one cheek of the guiding frame of each segment a silver plate of 
the same curvature is fixed, carrying engraved graduations reading to sino-le degrees. 
The movable segment itself, actuated by the milled-headed tangent screw, carries at 
the centre of its arc, and brought out to the side flush with the scale, a silver 
indicator upon which a zero mark is engraved. The graduations and the zero mark 
are so fine that, with the aid of a pocket lens, ten minutes of rotation can with ease be 
accurately estimated. The scale graduations commence from the centre and extend 
for a little over 35° on each side. Hence, when the segments are in the normal 
position, their ends flush with those of the guiding frames, the indicators point to 0° ; 
the amount of movement of either segment, brought about by rotation of the 
corresponding tangent screw, on either side, is consequently a.t once given by the 
scale-reading to which the indicator carried by the moving segment points. 
In addition to this pair of circular adjusting movements whose planes are fixed at 
right angles to each other, another pair is provided in which the planes of ch’cular 
motion may be arranged at any desired angle to each other. This alteinative 
adjusting arrangement is useful in certain rarer cases of crystals of monoclinic 
symmetry, in which faces are not developed which would lend themselves readily to 
the adjustment of the desired axis of optical elasticity by means of circular motions 
in planes at right angles, and also for use, if preferred, with triclinic crystals. The 
same centering arrangement is employed. It is oidy necessary to remove the ordinary 
rectangular adjusting apparatus by taking out the four screws which fix its brackets 
to the lower of the three centering discs, by means of a convenient screw-driver 
supplied, and to replace it by the alternative apparatus, the screw-holes in whose 
brackets are likewise made to correspond exactly wdth the tapped holes of the disc. 
Fig. 3 represents it in position. The lower circular movement is made capable of 
rotation in a horizontal plane about the upper, and the amount of rotation is registered 
by a silver divided horizontal circle fixed to the upper segment, and four indicators 
arranged at 90° apart carried by a disc rigidly attached to the guiding arc of the 
lower segment and rotating in close contact wdth the circle. In order to aftbrd room 
for the introduction of the two discs the upper segment is made of somewdiat larger 
radius than in the ordinary adjusting apparatus. The circle is divided, like the 
graduated arcs of the circular motions, into single degrees, and ten minutes can be 
