48o 
PHYSICS: DUANE AND STENSTROM Proc. N. A. S. 
In estimating the wave-lengths, etc., only those measurements may- 
be used that are made on the same date, for the zero of the instrument 
may have shifted between the dates recorded in the figures. 
FIG. 3 
The following table contains the wave-lengths, X, calculated from the 
average grazing angles of incidence, 6, obtained from all the measure- 
ments we have made in the spectra of the various orders, respectively. 
