600 PROCEEDINGS OP THE AMIrICAN ACADEMY. 



Practically the same thing occurs whea the interferometer is illuminated 

 with sodium light, except that in this case the density of the sodium 

 vapor in the optical path must be very much smaller. A periodic visi- 

 bility results even when the light of one pf the D lines is removed by the 

 polarizing system described in the previous paper. The case is of course 

 similar to the last-mentioned, for the width of the D line illuminating the 

 instrument is greater than the width of the absorption-baud of the rare 

 vapor. We thus have a condition identical with that which we had when 

 the emitting slit of the monochromatic illuminator was bisected with a 

 wire which cut out the D lines from the narrow baud of the spectrum 

 which was utilized. 



