124 
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 shortenthe focus. In 
this way it is possible to obtain very good results. He, 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 eecen- 
tric. A speculum thus polished shews « 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. 
