92 FRAGMENTS OF SCIENCE. 
position near the electric lamp, no neutral point was ob- 
served throughout the entire extent of the beam. 
When, this beam was looked at normally through tha 
selenite and Nicol, the ring-system, though not brilliant,, 
was distinct. Keeping the eye upon the plate of selenite,, 
and the line of vision perpendicular, the windows were 
opened, the blinds remaining undrawn. The resinous 
fumes slowly diminished, and as they did so the ring- 
system became paler. It finally disappeared. Continuing 
to look in the same direction, the rings revived, but now 
the colors were complementary to the former ones. Tlie 
neutral point had passed me in its motion down the beam,, 
consequent upon the attenuation of the fumes of resin. 
With the fumes of chloride of ammonium substantially 
the same results were obtained. Sufficient, however, has 
been here stated to illustrate the variability of the position 
of the neutral point.* 
By a puff of tobacco-smoke, or of condensed steam, blown 
into the illuminated beam, the brilliancy of the selenite 
colors may be greatly enhanced. But with different clouds; 
two different effects are produced. Let the ring-system ob- 
served in the common air be brought to its maximum 
strength, and then let an attenuated cloud of chloride of 
ammonium be thrown into the beam at the point looked at; 
the ring system flashes out with augmented brilliancy, but 
the character of the polarization remains unchanged. 
This is also the case when phosphorus or sulphur is burned 
underneath the beam, so as to cause the fine particles of phos- 
phorus or of sulphur to rise into the light. With the 
sulphur-fumes the brilliancy of the colors is exceedingly 
intensified; but in none of these cases is there any change 
in the character of the polarization. 
But when a puff of the fumes of hydrochloric acid, 
hydriodic acid, or nitric acid is thrown into the beam, 
there is a complete reversal of the selenite tints. Each of 
these clouds twists the plane of polarization 90 degrees, 
causing the center of the ring-system to change from black 
* Brewster has proved the variability of the position of the neutral 
point for skylight with the sun's altitude, a result pbviQttglv connoted, 
with |Ue foregoing experiments, 
