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 Eosse'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 Kosse's 6-feet 

 speculum. 



It is certain, however, that both machines have produced good 

 results in the hands of their inventors. 



Note. — In Mr. Grubb's Machine No. 2, one revolntion 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. 



