187 
ke/em? 9°.5) = (869 & 10—*)’. This agrees well with the value given 
by SIERTSEMA: (863 Xx 10—*)’. 
10. This led me to unite my results for the three gases with 
SIERTSEMA’s, and to plot the dispersioncurve for each for the 
visible and ultraviolet regions. The rotatory constant for the yellow 
mercuryline (mean wavelength 5780 A.U.) was taken to be unity; 
along the axis of abscissae are plotted wavelengths in wu, and 
along the axis of ordinates the following relative rotations: 
TASS thee MAL 
R‚IRsis 
ee 
578 ees LELIE) 1.00 1.00 
546 1.08 1.125 1.125 
486 1.265 1.435 1.44 
436 1.50 1.805 1.815 
404.5 1.655 2-10 2.15 
366.5 1.965 | 2.57 2.64 
313 2.695 3.76 3.90 
289.5 3.365 4.715 5.14 
265.5 3.755 5.49 5.975 
253.5 6.195 6.67 
248 7.09 
240 7.76 
238 7.94 
The dotted line at 423 wu gives the limit of SmrtsEmMa’s obser- 
vations. 
It is to be remarked that the oxygen-curve deviates considerably 
from those for the other gases, but that there is no sudden change 
as the ultraviolet absorptionregicn is approached, and that the 
difference between hydrogen and carbon-dioxide begins to be good 
appreciable in the ultraviolet region. 
On the assumption that ultraviolet refraction in hydrogen satisfies 
a formula of the type 
3 Proceedings Royal Acad. Amsterdam. Vol. XV. 
51 
