Si 



instruments. The mounting of the loo-inch re- 

 flector is making exceptional demands upon our 

 instrument makers and their shop equipment, 

 which has recently been enlarged to meet the needs 

 of this telescope and the still more exacting re- 

 quirements of a machine for ruling diffraction 

 gratings. 



FIG. 57. Machine Shop in Pasadena. 



The success of modern spectroscopic investi- 

 gation depends largely upon the size and excel- 

 ence of the optical gratings available. These 

 polished plates of speculum metal, ruled with 

 about 15,000 lines to the inch, have reached a high 

 degreee of perfection through the skill of Rowland, 

 Michelson, and Anderson. But certain types of 

 gratings required for our special work can not be 



