146 Royal Astronomical Society. 



the surface into equal squares, and the various manipulations which 

 are required to produce a perfect result. The grinding powder is 

 known as rouge, and the best quality may be had from Mr. Fox of 

 Saffron Hill. 



" The whole time occupied in obtaining the requisite lustre varies 

 from about one hour to three, and it ought to be steadily advancing 

 throughout. 



" A good idea may be formed of the quality of the operation as it 

 proceeds by watching the motion of the tool. It should be regular 

 and uniform, without any apparent labouring or inequality of speed, 

 and the spontaneous motion which the tool has upon the pin as a 

 centre should be slow and regular. No firm adhesion is ever to be 

 allowed between the tool and speculum : this will take place if a due 

 and regular supply of water be not afforded. 



" A second application of powder will rarely be required, and never 

 in any quantity, but many applications of water probably will, and 

 the more rapidly the polish is advancing the more frequently will 

 water be required. It is best applied through a hole in the back of 

 the polisher as near the centre as is convenient, which may perhaps 

 be at about the distance of one-third of the radius. But care should 

 be taken not to give the water in excess. The speculum must never 

 be dry, but there must be no superfluous water. It is very conve- 

 niently applied with a flat camel's-hair brush, half or three-quarters 

 of an inch broad ; but as much as the brush would take up would 

 generally be too much for one application. Towards the end, the 

 water should be added more sparingly, and if needful more fre- 

 quently, going as near to dryness as may be but never reaching it. 



" The lustre in this state of the process advances most rapidly. 

 If the process has gone on well the powder will have become almost 

 black at the close. The machine having been stopped, the tool is 

 to be carefully taken off by a sliding motion, and the speculum may 

 then be cleaned with a soft linen cloth or leather ; or it may be 

 washed with a soft sponge and water, and then dried, and ultimately 

 rubbed lightly with some very soft wash-leather. If the polishing 

 has apparently wrought smoothly, and the aspect of the tool when 

 taken off, both during the process and at its close, is everywhere of 

 even texture when viewed by an oblique light, the speculum will 

 most likely have a uniform curve of some description, whether para- 

 bolic or not, for it is a characteristic quality of this machine gene- 

 rally to produce a uniform curve. The quality of the curve is best 

 examined by placing the mirror in its tube, and, by means of dia- 

 phragms, exposing separate portions of the mirror of equal area 

 from the centre to the circumference. 



" I have been accustomed to produce by hand surfaces of, I believe, 

 great excellence, on various sized specula up to nine inches diameter, 

 of which I may instance my 9-foot equatoreal, which enabled me to 

 discover independently (for I did not previously know of its existence) 

 the sixth star in the trapezium of Orion, and with which also the 

 observations of a second divison of the ring of Saturn were made, as 

 described in the Astronomical Notices, vol. vi. p. 11. Such sur- 



