On Apparatus for Polishing Specula for Reflecting Telescopes. 109 
may be caused to describe, that it probably deserves the pre- 
ference. On the other hand, on looking at figure No. 5, and 
observing the variety of continuously varying curves described 
by the polisher in Lord Rosse’s machine, one has little doubt that 
with it there will be less liability to polish the surface in rings 
than with a machine which gives a uniform curve of whatever 
description. 
The objection that the polisher is kept too long over the edge 
by the eccentric (G, figure 4), is obviated by making the pully by 
which it is driven oval as in the machine for Lord Rosse’s 6-feet 
speculum. 
It is certain, however, that both machines have produced good 
results in the hands of their inventors. 
Nore.—In Mr. Grubb’s Machine No. 2, one revolution of the speculum coincides 
with 14 strokes of the eccentrics, of which there were 33 per minute in the rough 
grinding, and 24 in fine grinding and polishing. 
