310 
ME. W. LASSELL ON POLISHING THE 
of the hones in the same direction*. When the coincidence has been rendered nearly 
perfect, the tool may be wrought upon the speculum with a little tine-sifted emery, 
water being freely applied as the mud forms, and finishing with finely powdered hone- 
dust. This process will produce a very fine surface on the speculum, quite fit for the 
application of the polisher and for examination of the figure by the image of a bright 
star. This tool is very convenient in case of having, in polishing, produced a hyper- 
bolic figure, as it may be reverted to and a spherical figure obtained in an hour’s 
working. On the back of the bed of hones, as also on the back of the polisher, is 
screwed a cast-iron socket, loosely fitted by a stout pin, about half an inch in diameter, 
depending from the lever or long arm (C, fig. 8). This pin should be firmly attached, 
as it has to bear all the strain of the machine, both in grinding and polishing. The 
weight of the bed of hones is about 61 lb., and of the polisher about 35 lb. 
Presuming the speculum to have now a sufficiently fine and approximately spherical 
surface from the hone-tool, it will be ready for the polisher. The temperament of the 
latter should be of course in due relation to the existing temperature of the laboratory. 
The surface of the pitch must retain its originally pure texture, or it will not polish 
quickly and well ; and it must be slightly warmed and placed upon the clean wetted 
face of the speculum before any powder is applied, to insure a nearly even and uniform 
contact between the polisher and speculum. 
The surface of pitch is conveniently and uniformly warmed by the apparatus repre- 
sented in fig. 13. Two pulleys revolve on axles driven horizontally into a beam, a stout 
cord (sash-line) running over both. One end of the cord is attached to a counterpoise- 
weight, and the other by a swivel and three cords to the polisher, which is thus suspended 
face downwards. The three cords terminate in three hooks, respectively receiving the 
ends of the three equidistant pins inserted in the circumference of the polisher. At 
a is a piece of wood and tightening-screw, which can be made to clip the cord to the 
beam, and prevent its motion when the equilibrium is about to be destroyed by removal 
of the polisher. When in cequilibrio the polisher can be raised or lowered at pleasure, the 
screw being withdrawn during the process of warming. On the floor, under the polisher, is 
placed a small chauffer or furnace (Plate 52. fig. 12), made of four fire-bricks or tiles 
9 inches long, 4^ broad, and 2^- inches thick. These are put together so as to enclose a space 
about 5^ inches square and 4^ inches deep, which forms the receptacle for the charcoal. 
The base on which the bricks stand contains a grating of bars ^ inch square, with equal 
spaces between. Supported on the bricks, and a few inches above them, is a piece 'of 
sheet iron rather larger than the area of the furnace, to prevent the direct heat of the 
ignited charcoal from acting on the pitch. The heated air ascends all round the plate, 
and by revolution of the polisher with a little swinging motion, its surface is uniformly 
warmed. By means of the counterpoise, the polisher can be raised or lowered at pleasure 
according to the heat required. Some judgment is necessary in warming the pitch. 
* The hones are put on entire in their whole length ; but their upper surfaces are slightly grooved, so as to 
give the tool the aspect of a system of squares. 
