902 
MR. A. B. TUTTOR OB' AN INSTRUMENT FOR GRTBTDING- SECTION-- 
aii angle of 90°-120° from it, and the goniometer lamp in front of the slit. The zone 
of faces is then adjusted in the ordinary goniometrical manner with the aid of the 
centering and adjusting movements, so that the images of the vertically-arranged slit 
reflected from the various faces of the zone are bisected by the horizontal spider-line 
upon rotation of the crystal and all that moves with it by means of the ebonite 
milled disc. The plane which it is desired to grind will then be parallel to the 
grinding disc. Even in this simple case the graduations of the circle are valuable, as 
enabling the observer to make quite certain, by taking the angular distances of the 
faces, that the adjusted zone is really the one which it was desired to so adjust. 
During these operations, any vertical motion of the crystal, in order to raise or 
lower it to the height of the axis of the optical tubes, is brought about by movement 
of the inner steel axis r by means of the milled head at the top of the axis, the gun- 
metal axis i being fixed, the elbow p of the lever carrying such being kept down upon 
the circle plate. 'I'he other lever should be adjusted to rest approximately horizontally 
by suitably arranging the screw o. It will now be found that while the elbow of the 
front lever rests upon the circle, its short curved arm is alone supporting the axis, the 
terminating blunt knife-edge of the short arm of the hoi-izontal lever being, perhaps, 
a quarter of an inch below the collar fixed to the axis. By allowing the elbow lever 
to rise, gently assisting it at first, the axis falls until at length its collar likewise rests 
upon the knife-edge of the horizontal lever, when any further downward movement of 
the axis occurs with practically the whole weight counterbalanced by both levers. 
About this point the weight above the crystal can be varied almost to any extent, 
according to the judgment of the manipulator as to the strength of the crystal. The 
grinding plane should now be fixed in position over the polishing plane, and a few 
drops of sweet oil placed upon it. The oil should be evenly distributed over the 
marginal portion of the grinding surface, where the grinding occurs, by means of a 
camel-hair brush, carried by a small movable stand ; the brush also serves the purpose 
of sweeping the plane in front of the crystal. 
The inner steel axis r is then lowered by means of the upper milled head until the 
crystal is not more than an eighth of an inch above the grinding surface, keeping the 
left hand upon the lever so that its elbow still rests upon the circle plate. The lever 
is then gently assisted upwards, its rapidity being kept under full control until the 
crystal just touches the grinding disc, when rotation of the latter may be commenced, 
very slowly at first. If the crystal is not extremel}^ friable the horizontal lever mav 
be allowed to remain out of action till the grinding is nearly finished, for the collar of 
the axis will still be more than one-sixteenth of an inch hig-her than the knife-edge of 
that lever. The maximum pressure on the crystal will therefore be equal to about 
half the weight of the axis, and a very large number of artificial crystals will not 
break under this pressure. The weight with which the crystal bears upon the 
grinding disc can, however, be beautifully regulated by gently holding the counter¬ 
poise of the lever between the thumb and first finger of the left hand, steadying the 
