354 



PROCEEDINGS <>F THE AMERICAN ACADEMY. 



It is difficult to see how non -parallelism of slit and ruling could in 

 this case introduce a shift. Keller seems to have overlooked the fact 



Figure 1. 



AA', direction of grating rul- 

 ing; KK', direction of slit; LL', 

 direction of resultant line. 



I 



F 



FlGUBB 2. 



A A', two exposures of an arc line 

 superimposed horizontally, but not 

 vertically; FI", spark tine. 



that this triple method was employed, for no mention is made of it in 

 his piper. However, despite the fact that it was not apparent how 



