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 ^TFro of an 

 inch (ytts^)- 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. 



