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described by Lord Rosse in the Philosophical Transactions. 

 It makes the speculum revolve once for twenty-four and a-half 

 strokes, and theeccentric once for eighteen. Fromseven toeight 

 strokes of twenty-four inches are made in the minute, and the 

 lateral movement of the speculum by the eccentric is fourteen 

 inches.* A screw whose nut runs on a railroad above the 

 machine lifts the speculum, with its frame and levers, from the 

 truck, and deposits it on the revolving platform, where it is 

 levelled, centred, and secured. The same apparatus serves 

 to move the polisher during its preparation and to apply it to 

 the speculum, so that it is even more manageable than that of 

 the three-feet was. It was cast with the transverse grooves ; 

 the circular were cut in the lathe. The time required for 

 polishing is about six hours ; and Lord Rosse has found that 

 this period cannot be exceeded without injury to the figure, in 

 consequence of the soft pitch being squeezed out, and the 

 harder and unyielding material coming into contact with the 

 iron of the polisher: unfortunately, this occurred to some extent 

 in the present instance. The ammoniacal solution of soap used 

 towards the close of the process, happened to be made with 

 ammonia prepared from gas liquor and containing some sub- 



* These are the proportions which Lord Rosse prefers; but it must be 

 kept in mind that they change with circumstances. Probably they will not 

 answer for those specula which have an aperture larger than one-ninth of 

 their focal length, and certainly not for those which are perforated in the 

 middle. Dr. Robinson has made many experiments on one of the latter, fifteen 

 inches aperture and nine feet focus, with a machine nearly the same as Lord 

 Rosse's ; and he finds that the nature of the polishing depends on the figure 

 given in grinding. If the eccentric be regulated so as to make this hyper- 

 bolic, its action must be lessened in polishing so as to shorten the focus. In 

 this way it is possible to obtain very good results. IJe, however, prefers the 

 opposite course pointed out by Lord Rosse ; grinding to an elliptic figure, 

 he polishes with a very long primary stroke, and small action of the eccen- 

 tric. A speculum thus polished shews t Arietis well separated and defined 

 with 940, and with 465 the fifth star in the trapezium of Orion's nebula is 

 visible even when the acting surface is reduced to seventy-two circular 

 inches. 



