Physics, 261 
right angles to its own length was perfectly polarized. By degrees 
the cloud became of whitish blue, and for a time the selenite col- 
ors obtained by looking at it normally were exceedingly brilliant. 
irecti disti i 
long as the cloud maintained a decided blue color, and even for 
some time after the pure blue had changed to whitish blue. But 
as the light continued to act the cloud became coarser and whiter, . 
particularly at its center, where it at length ceased to discharge 
polarized light in the direction of the perpendicular, while it con- 
tinued to do so at both its ends. 
But the cloud which had thus ceased to polarize the light emit- 
ted normally, showed vivid selenite colors when looked at oblique-. 
y. The direction of maximum polarization changed with the tex- 
ture of the cloud. This point shall receive further illustration sub- 
sequently. 
an angle of 90° with the axis of the illuminating beam. __ 
It is exceedingly interesting to observe both the perfection and 
S 
some time after the cessation of perfect polarization the resid- 
mental tube. h 
hitrite of butyl were introduced into the tube, and they were fol- 
lowed by about 3 inches (measured by the mercurial gauge) of air 
* This seems t that particles too to polarize the blue, polarize per- 
fecty light of lower rekangiity, 
