SPECULA OE REFLECTING TELESCOPES. 
311 
The heat must not penetrate far into it, nor must the heat be so suddenly or powerfully 
applied as to r^elt the surface ; it must be merely softened. The surface of the speculum 
having been freely wetted by a large sponge and clean cold water, the pitch-surface as 
softened must be quickly laid upon the speculum and gently and slowly moved, to faci- 
litate contact. It is well not to err on the side of too much warming, as if necessary the 
process can be repeated until the contact is complete. On removal of the polisher it will 
be instantly seen to what extent the surfaces coincide ; and it is desirable that the contact 
should be very uniform, and that some part at least of every square should touch the 
speculum ; if that is not the case, another warming should be resorted to. For polishing 
I use th q finest plate-powder, or jeweller’s rouge, which may be obtained of excellent 
quality from Medway and Co., Owen’s Court, Goswell Eoad. This requires no further 
sifting. A quantity of it is put into a flat-bottomed jar and well stirred about with 
water equal to seven or eight times its bulk. It is then left to subside until almost all 
the water can be poured off quite clear. Of course the finest particles of the powder 
will be now upon its upper surface ; and I have ever found these to be capable of pro- 
ducing as fine a lustre as the speculum is capable of receiving. Without disturbing 
much more than the surface of the powder, the speculum is now, by means of a flat 
camel’s-hair pencil, to be covered with the rouge and water of the consistence of cream. 
The polisher may be again very slightly warmed, placed upon the speculum and the 
machine set to work. 
The motive power I have used and found quite adequate is a steam-engine of 4^ 
inches diameter of cylinder and 8 inches stroke, making 120 revolutions per minute, 
with pulleys on the horizontal shaft of such size as to drive the crank H either thirteen 
or seven revolutions per minute. 
The mode I have employed for converting the spherical curve into a parabolic one 
has been by flattening or wearing down the exterior parts of the mirror, not by deepening 
the central ones ; and this has been accomplished generally by altering the throw of the 
crank H and its rate of motion. In this I have been guided by the following consider- 
ation. If the polisher, at a proper temperament, be placed out of the centre upon the 
speculum with one part of its edge hanging over the speculum some 3 or 4 inches, and 
be allowed to remain a few minutes in that position, it will require a strong effort to 
bring it back to a central position over the speculum, the unsupported pitch-surface 
having in that interval fallen down slightly by the force of gravity. In forcing the 
pitch back to its normal figure, it must necessarily exert an undue pressure upon the 
exterior part of the speculum over which it travels, and, if charged with powder, would 
certainly abrade that part more than other parts nearer the centre, and, so far as its 
operation reached, would tend therefore to produce a parabolic curve. Now if the 
throw of the crank H be lengthened, the polisher at each stroke will be carried more 
over the edge, and therefore this tendency would be increased ; and if at the same time 
the speed be diminished, the duration of the overhanging of the polisher at each stroke 
would be longer, and the parabolic effect proportionally increased. The result is still 
2 t 2 
