142 
DE. T. E. EOBINSON AND ME. T. GEUBB’S DESCEIPTION OF 
the cranks H, and of the length of the rods I, a motion can he produced varying from 
nearly a straight line through a series of ovals to a circle of any extent, and either con- 
centric with, or excentric to, the underlying speculum. 
The vertical bar K passes at its upper end through a hole in a bar attached to the 
roof of the building, and besides producing the horizontal motion mentioned, performs 
another important office. 
To the lower end of the bar attaches, by a cross-key, a flanged socket which supports 
the triangular piece I, and at its upper end the bar is attached to a lever with adjustable 
weights, so that a pull upwards in the direction of the bar to any desired amount is pro- 
vided, and thus any portion of the weight of the grinding- or polishing-tool in excess of 
that desired is relieved. 
The figure shows the details of the system of support as applied to the grinding-tool. 
The central piece L being upheld by the bar K, supports the three straight bars M, and 
these again support the six triangular pieces N. 
Thus any portion of the entire weight of the tool is supported equally from eighteen 
points. The following arrangement was provided for the quick and convenient trial of 
the speculum while undergoing the process of polishing. To the hollow beam B is 
attached the apparatus O, P, Q, O being a massive toothed sector, P an endless screw 
working in same, and Q a wheel and pinion, with a hand-winch. This part of the 
apparatus serves either to retain the speculum in an horizontal position for polishing 
&c., or for quickly bringing it to the vertical, when by the opening of those doors, pur- 
posely provided in the building, the speculum can be tested upon a day object sufficiently 
distant ; and again by a reverse motion of the winch the speculum is as quickly restored 
to the horizontal position for continuing the process if required: this facility of testing 
the figure is of the highest importance to the ease and certainty of the polishing ; with- 
out it one would be working in the dark. 
As it was deemed necessary to grind both back and face of each speculum, two grinders 
were provided ; the form of the back of these is shown in the figure ; both grinders were 
cast with grooved surfaces, forming projecting squares of 3-inch with ^-inch spaces, and 
prepared in the lathe, one of them flat for grinding the back, the other to the required 
curvature by a slide-rest actuated by a guide to the intended radius ; each speculum was 
treated as follows. 
First, the face was ground to a good surface and approximately to the required curva- 
ture, then reversed and ground flat on the back ; in both cases the speculum rested in 
the face-plate of the grinding-machine with felt interposed, and thus free access was 
obtained to the edge of the speculum for the accurate trueing by grinders of the pro- 
jecting band. Next, the speculum was once more reversed and placed in its box upon 
its ultimate supports, from whence it was not disturbed during the subsequent operations 
of fine grinding and polishing. Before commencing the fine grinding of the face, the 
grinding-tool was, by cross-cutting the squares on the face, formed into squares of about 
If inch each, and by a little contrivance and some care the system of supporting-levers, 
