Royal Astronomical Society. liS 



been accustomed to produce very satisfactory surfaces on smaller 

 specula." 



The machine invented by Mr. Lassell, and constructed by Mr. 

 Nasmyth, for figuring and polishing specula, cannot be made intel- 

 ligible without figures*. The speculum rests with its face uppermost 

 in a horizontal position, and is carried slowly round by a vertical 

 axis. The polisher rests, with its grinding and polishing surface, 

 upon the speculum, and is moved by a pin which fits loosely into a 

 hole in the centre, at the back of the polisher. The motion of the 

 polisher is that of the driving-pin. 



Now this, by a very ingenious and very compact mechanism, re- 

 ceives a compound motion which may be thus imagined. Conceive 

 a circular motion given to a point round the centre of the speculum, 

 and then conceive that the driving-pin has a circular motion round 

 this point. The curve is an epitrochoid, and the adjustments of the 

 mechanism enable the workman to give any radius to either circular 

 motion, from up to a certain number of inches. The proportions 

 of these radii, in order to give a parabolic figure, are determined ex- 

 perimentally, in which the relation of aperture to focal length must 

 be considered. The size of the polisher, and even the hardness of 

 the pitch, must also be proportioned to the figure and aperture re- 

 quired. Mr. Lassell finds no difficulty in getting a true parabolic 

 figure when the aperture is one-eighth of the focal length f. The 

 speculum, while grinding and polishing, is supported in the same way 

 as it is in the tube when in use. The principle of this mode of sup- 

 port is mentioned by Lord Rosse, Philosophical Transactions, 1840, 

 p. 524. 



"The polisher should possess as much stifl'ness as is compatible 

 with the requisite lightness, and I have found these qualities best 

 combined by making it of white American deal, in two strata, well- 

 united by glue and a few screws, with the direction of the grain at 

 right angles, the wood well-seasoned, and, if possible, cut out of 

 the same board. The polisher for the 2-foot speculum is made out 

 of \\ inch board, and has, for symmetry, both the upper and under 

 surfaces convex, to fit the speculum. It is about 2 inches thick at 

 the circumference, '2Q\ inches diameter, and weighs about 12 lbs. 

 with the pitch surface upon it." 



Mr. Lassell then enters very minutely into the mode of coating 

 the polisher with pitch evenly and to a proper thickness, of dividing 



* A model to half the true size, and tiie drawing by Mr. Nasmyth, may 

 be seen at the Society's apartments. A model of Lord Rosse's engine, and 

 of the mounting, &c. of the B-foot reflector at Birr Castle, may also be 

 seen. These were made by Mr. Airy, and |)resented by him to tlie Societj'. 

 Mr. Williams will explain the action and details of all the models to any 

 fellow who wishes for information. 



\ Mr. Lassell has given so full an account of all his processes that we 

 conceive any person of ordinary intelligence would be able to execute 

 them ; but they do not admit of compression, and extend beyond the limits 

 of a Monthly Notice. It ought to be mentioned that the Eail of Rosse 

 and Mr. Lassell have at all times freely conmiunicated the steps of progress 

 as soon as these became evident to themselves. 



Phil. Mas, S. 3. Vol. 34. No. 227. Feb, 1849. L 



