214 Lord Oxmantown on an Apparatus for grinding and 



the process, the speculum may be made to revolve with consi- 

 derable rapidity by altering the arrangement of the bands com- 

 municating the motion ; and the tool NO, for the same pur- 

 pose, may be prevented from turning by a pin screwed into it, 

 which catches against the rod TU. Coarse emery is now in- 

 troduced between the tool and speculum through a hole in 

 the tool ; and the reciprocating motion of NO, combined with 

 the rotatory motion of LM, will soon bring the speculum to a 

 good surface, though not truly spherical. The focal length 

 of NO being now a little changed, it is replaced by another 

 tool composed of lead and tin, and NO is now suffered to re- 

 volve with the speculum, the pin that prevented it being re- 

 moved ; besides the bands are now arranged as in the figure, 

 to give a slow motion to the speculum, and emery of different 

 degrees of fineness is made use of, till the surface of the specu- 

 lum is extremely smooth. 



No fresh emery should now be added for at least a quarter 

 of an hour ; and the speed of the shaft CD, and of course of 

 the whole apparatus, is reduced during that time one-half. This 

 seems favourable to the production of a very accurate spheri- 

 cal surface. 



The speculum will then be ready to be polished, for which 

 purpose the tool is to be covered in the usual manner with a 

 coat of pitch of the proper consistency, and the usual polish- 

 ing powder applied. At the commencement the shaft CD 

 may revolve with its ordinary speed, and a considerable weight 

 may be applied to the spring RS ; this will almost immediately 

 bring the pitch exactly to the same curve as the speculum, and 

 the polish will rapidly proceed ; however, towards the end of 

 the operation both the speed of the apparatus and the weight 

 must be greatly diminished. 



It appears, then, that the friction by which the polishing 

 and the latter part of the grinding is effected arises from the 

 reciprocating motion of the tool. The circular motion of the 

 speculum is merely for the purpose of continually altering the 

 direction of the strokes resulting from the reciprocating mo- 

 tion. The tool will not revolve exactly in the same time as 

 the speculum ; this is to a certain extent useful ; but would be 

 prejudicial if friction were produced by it amounting to any 



