

polishing the Specula of Reflecting Telescopes. 215 



sensible quantity. It is principally for the purpose of obviat- 

 ing this that the speculum is made to revolve so slowly. I 

 have tried the introduction of another motion into the process 

 by fixing the guides YY on eccentrics revolving very slowly, 

 and having a very small eccentricity. This seems to be an 

 improvement ; but as it renders the machine more complicated, 

 and as I have not yet tried it sufficiently to be able to speak 

 with certainty of its good effect, I have not represented it in 

 the figure. 



It is evident that a considerable number of specula or lenses 

 may be worked at the same time, by increasing the number of 

 the wheels Z. I have not, however, extended the apparatus 

 beyond what the figure represents, having no occasion for it. 



The annexed sketch has no pretensions to the accuracy of a 

 working drawing. The scale is merely for the purpose of giv- 

 ing a general idea of the proportions of the parts : The machine 

 would be sufficiently large to grind and polish a speculum of 

 three feet diameter or perhaps larger. Should an instrument- 

 maker think it worth his while to construct an apparatus on 

 the above principle, probably one on one-third of the scale 

 would be sufficient for his purposes ; — it might be moved by 

 men and a porter's wheel. The quantity of power necessary 

 would of course depend upon the velocity of the machine, 

 and the size and number of the specula to be worked. 



The engine I make use of in my laboratory is a two horse 

 power ; and from some loose experiments with it, I should 

 think that a one horse power would be fully sufficient for three 

 or four specula six inches diameter. The rod T U may make 

 a stroke of about an inch for a speculum of the above dimen- 

 sions, and the revolutions of the speculum may be to the re- 

 volution of the eccentric as 1 : 300 or 400. For specula of 

 six inches diameter a day will be found sufficient to complete 

 the process. 



It is evident that the object of this apparatus is to commu- 

 nicate an accurate spherical figure. Should it be proposed to 

 attempt the parabolic curve, the motion recommended for 

 that purpose might be imitated by means of the eccentric 

 guides and the slow circular motion of the speculum ; and 

 with this advantage, that, were it found really successful, the 

 same result would probably be always afterwards obtained ; 



