SPECULA OF REFLECTING- TELESCOPES. 
313 
surface as before. A very slight application of only the finest particles of fresh powder 
is now to be applied to the speculum and the process renewed. Before this powder 
becomes used up, the lustre ought to be sufficient, as may be ascertained by inspecting 
the circumferential parts of the speculum as they become successively exposed. 
The tendency to dry at the edge is now much increased, and must be prevented by 
touching the edge from time to time with the camel’s-hair pencil charged with scarcely 
coloured water. This may be continued for some time, the lustre advancing very 
rapidly, and the colour of the powder becoming really black. What I have described, 
if all has gone on well, may occupy perhaps three hours, speaking roughly ; for the 
perfect success of the process depends upon some niceties — such as the exact adjustment 
of the temperament of the pitch to the heat of the apartment, and the steadiness of its 
heat during the time of working. I may remark that nothing abnormal in the process, 
such as any irregularity in the motion of the polisher on its axis, must be permitted to 
occur without at once stopping the engine and correcting the error ; perfect smoothness 
and evenness of motion must exist throughout, and also perfect uniformity of aspect of 
the polisher in its several removals must be apparent, or a good result cannot be hoped 
for. But in this machine the motions of the polisher are so perfectly, and yet, so to 
speak, so gently controlled, that irregularity is not to be expected ; and if it should 
occur, a moment’s reflection will most likely reveal the cause. It is probable that 
this first polishing may not have brought the speculum quite up to the parabola, but 
I think it desirable to clean it off and prepare it for examination. This is best done 
with a large and very soft sponge and plenty of pure water. This cleansing must be 
continued until not a vestige of powder remains, nor any trace of smear or impurity 
whatever. The sponge having removed all superfluous water, the drying is to be 
effected with a couple of old soft linen cloths, good absorbents. The wiping must 
be light and quick, and continued until every particle of moisture has disappeared 
from the speculum. It is then best left without a cover for half a day, that it may 
uniformly acquire the exact temperature of the apartment. The cover may then be 
put on, and afterwards the less the surface of the speculum is touched the better. 
If there he occasion for it, however, I never hesitate to wash and wipe it in the same 
soft and careful manner; nor indeed, if at any time there appear any grease-stains 
or spots, to first pour upon the surface a very little soft water in which a bit of soda 
has been dissolved, lightly distributing it over the whole surface with the finger, and 
immediately afterwards applying the large sponge and pure water. 
The polisher also requires immediate attention* as if no great change in the tempe 
rature of the air occurs, it may be used for repeated polishings. But the surface 
charged with rouge and metallic particles must be completely removed or it will never 
polish well. This is best done by first well washing, to remove all that a painter’s brush 
will remove, and then tearing up the extreme film of the surface with a piece of “ steel- 
card,” such as is used in cotton-mills for carding cotton. As this film is extremely hard 
and rapidly destroys the edge of any cutting-tool, it was a relief to me to find that this 
