SPECULA OF REFLECTING TELESCOPES. 
309 
square is applied. I begin at the centre, as in the figure ; and after laying down two 
rows along the two radii of the first quadrant, I then proceed to the opposite quadrant, 
and similarly with the remaining quadrants, with a view of producing uniformity as 
much as possible over the whole surface. The squares are now to be successively taken 
up by the fingers, drawn rapidly across the heated plate ( a , fig. 11), which completely 
melts the under surface of the square without penetrating beyond a mere film, and 
prepares it to adhere firmly to the base by a pretty hard but quickly withdrawn pres- 
sure. The 24- and 12-inch rods should be removed as soon as they have answered 
their purpose, lest any of the squares should adhere to them. If not used immediately, 
the polisher should be kept in a strictly horizontal position, either face up with a cover 
on, or inverted and suspended by hooks embracing three pins of stout wire, inserted 
equidistantly in the circumference of the polisher, as in figure 13. 
Before describing the actual process of polishing, I may say a word or two on the 
rough-grinding and preparing the speculum for reception of the polisher. The rough- 
grinding proper is a very easy process, and may be accomplished in various ways, the 
chief requisite being patience. A very good grinder may be constructed exactly as is 
the base of the polisher, and then covered with 2-inch or 2^-inch square leaden castings, 
four or five tenths of an inch thick, each screwed to the base by a couple of stout 
joiner’s screws. A convenient mould for the castings may be very easily constructed of 
sheet iron, with pins inserted to leave holes for the screws. The metal is improved if 
a little tin be added to the lead. Of course, the process of grinding must be watched, 
and the gauge of curvature applied occasionally, correcting any error by lengthening or 
shortening the strokes of the machine as the case may require. In this way, by 
gradually increasing the fineness of the emery, the surface of the speculum may be 
brought up to a condition fit for the polisher ; but, finding the process very tedious 
towards the last, and having been frequently much annoyed by the sudden appearance 
of a scratch or two, I have resorted to a bed of hones , as an intermediate tool between 
the grinder and polisher. 
The base of this is a circular disk of Bangor slate, 24 inches in diameter, and about 
eight tenths of an inch thick, planed flat on both sides. This is covered with pieces of 
German hone (Bohemian blue stone) ; they are to be obtained from F. Alexander, 
103 Leadenhall Street. The hones are about 7 inches long, and about eight tenths of 
an inch square. They are cemented on to the base with hard pitch, their under 
surfaces having been previously ground flat on a facing-plate, as it is necessary that 
their contact with the base should be intimate and accurate. The upper surface of 
the bed of hones must of course be made to fit the gauge of curvature, which is accom- 
plished, without much difficulty, with a coarse file or rasp, correcting it as the coincidence 
approaches accuracy by a few strokes upon the speculum itself. Fig. 14 represents 
generally the form of the bed, and the direction in which the hones are placed, attention 
being paid to balancing, so to speak, the opposite sides of the tool by having the grain 
MDCCCLXXV. 2 T 
