216 Lord Oxmantown on an Apparatus for grinding §c. 



but, from reasons which I have stated in an account of an at- 

 tempt to improve the reflector, (see last Number, p. 25.) I fear 

 the parabolic curve is inconsistent with an accurate surface. 



In Fig. 3, AB represents a lathe for cutting the tools for 

 grinding specula and lenses. The mandril works between 

 brass jaws secured by a variety of screw bolts to prevent any 

 shake. It is more to be depended upon than one of the com- 

 mon construction, and admits of easier adjustment. CD per- 

 forms the office of the slide puppet of a common lathe, but is 

 steadier, and in other respects preferable to it. The triangu- 

 lar bar EF is moved backwards and forwards by the screw G. 

 IH is a light frame of wood. K the cutting edge. IM a 

 rod of iron made flat at I and pierced with a small hole. The 

 rod IM may be drawn out more or less ; and then it can be 

 secured by a screw. 



To cut a convex tool, the lathe AB and puppet CD are 

 placed upon the bed of the lathe, Fig. 2, A and F being turn- 

 ed towards each other. The axis of the mandril AB and of 

 the triangular bar CD are then brought into the same right 

 line, and placed at the proper distance. The frame IH is 

 then adjusted so that the interval between the hole at I and 

 K shall be exactly equal to the radius of the tool to be cut. 

 The extremity of the bar IM is then inserted into the hori- 

 zontal slit in the bar EF, and then secured by a steel pin, up- 

 on which it plays, and the edge K is carried by a slide rest 

 along the surface of the tool, which had been previously fixed 

 on at B, and in a horizontal line corresponding with its dia- 

 meter ; and the impression it makes may be regulated by the 

 serew C. A very accurate spherical surface of exactly the re- 

 quired radius will thus be formed. A concave tool is cut in 

 the same manner, only the lathe AB is reversed, and a simple 

 radius is substituted for the frame IH. 



I have cut tools with this of twelve feet radii, but shorter 

 radii are more manageable. 



The whole of the above apparatus has been very lately con- 

 structed. When it has been more used, improvements will no 

 doubt suggest themselves. The interposition of another wheel 

 and spindle between VW and LM would be of use ; as it is 

 rather difficult without it to communicate a sufficiently slow 



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