Figure 17. — Machine used by Alvan Clark & Sons for polishing the 

 36-inch objective for the Lick telescope. While the lens is evenly 

 rotated, a reciprocating motion is imparted to the tool by the two 

 horizontal arms. A grooved pitch lap lies on the floor in front of 

 the machine. From cover of Scientific American, 24 September 

 1887. 



284-281 O- 



59 



