4 SMITHSONIAN INSTITUTION 



to deflect the recording spot of light (whose lever arm, as just ex- 

 plained, was nearly 20 feet) by less than -jV inch. Nevertheless, so 

 beautifully quiet and free from accidental disturbances was the record- 

 ing- spot that in successive determinations of spectrum energy on 

 different nights the agreement was usually excellent, and the diver- 

 gences hardly exceeded the thickness of a sheet of paper on the scale 

 of 20 feet lever arm. 



In all this work the aid and encouragement of Dr. Adams of the 

 Mount \Mlson Observatory was invalual^le. 



