On Apparatus for Polishing Specula for Reflecting Telescopes. 99 
is so small that at the margin A of a telescope 4 feet in diameter 
and 40 feet focus, their distance apart is less than the ;45,5 of an 
inch (gy$s3). You will observe that the parabolic figure is 
best suited for parallel rays, such as come from the heavenly 
bodies ; for viewing terrestrial objects the spherical form is just 
as good, and for near objects, much better. 
After two years’ interruption, caused by the Great Plague, Sir 
Isaac Newton, to use his own words, had then “thought on a 
tender way of polishing proper for metal,” and set to work. 
After a time he succeeded in making the first reflecting telescope 
of thirteen inches radius, by which he was able to see Jupiter’s 
four concomitants, as he calls them. And strange to say, since 
then we have simply been following in his track; he indicated 
the metals still used in the construction of the speculum, and the 
mode of operation we owe to him in a great measure; we have 
only improved in the details. 
The production of a spherical surface is comparatively easy, for 
the mutual rubbing of two bodies naturally tends to produce that 
form, but it is otherwise with the parabolic. 
We will first describe in detail the hand process of polishing 
specula used up to the time of Sir Wm. Herschel, who first con- 
structed a machine for this purpose. 
A tool made of iron, pewter, or some such material, was cast 
and turned to a radius of twice the intended focal length. Some 
had this tool of a greater diameter than the speculum, some of 
the same diameter ; this was fastened on an upright post, with 
the face upwards, emery and water was applied, and the operator, 
holding the speculum by a wooden handle cemented to its back, 
walked round the post, pushing the speculum to and fro in 
straight, elliptic, or circular strokes, supplying emery from time 
to time, until every part of the speculum was acted on equally by 
the emery. The tool was then examined to see if the curvature 
had altered, if so it was turned again to the proper radius, and 
the grinding proceeded as before. When the speculum was 
equally acted on by the emery, and the tool found to be of the 
proper curve, finer emery was applied, and finally using only the 
sediment obtained from water in which flour of emery had 
been stirred up and allowed to stand for ten seconds, thirty 
seconds, and up to four minutes before being poured off. 
