Vol. 6, 1920 PHYSICS: DUANE, FRICKE AND STEN STROM 609 
estimated with reasonable certainty to within about 10" of arc, and, as 
B equals 61' in this case, the error of precision amounts to less than one- 
third of one per cent. 
The other curves in figure 1 and the curves in figure 2 belong to mer- 
oury and bismuth, respectively. One pair of curves represents the crit- 
ical absorption of bismuth in the second order spectrum, and another 
pair, the critical absorption of mercury in the third order. The drops 
corresponding to the absorption in the second and third order spectra 
are very much smaller than in the first order, and for this reason it is 
