espectively.

After the clouds of gunpowder had cleared away, the laboratory was
filled with the fumes of common resin, rendered so dense as to be very
irritating to the lungs. The direction of maximum polarisation
enclosed, in this case, an angle of 12 deg., or thereabouts, with the axis
of the beam. Looked at, as in the former instances, from a position
near the electric lamp, no neutral point was observed throughout the
entire extent of the beam.

When this beam was looked at normally through the 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 colours were complementary to the former
ones. _The 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. [Footnote: Brewster has proved the variability of the position
of the neutral point for skylight with the sun's altitude, a result
obviously connected with the foregoing experiments.]

By a puff of tobacco-smoke, or of condensed steam, blown into the
illuminated beam, the brilliancy of the selenite colours may be
greatly enhanced. But with different clouds two different effects are
produced. Let the ring-system observed 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
polarisation remains unchanged. This is also the case when
phosphorus, or sulphur, is burned underneath the beam, so as to cause
the fine particles of phosphorus or of sulphur to rise into the light.
With the sulphur-fumes the brilliancy of the colours is exceedingly
intensified; but in none of these cases is there any change in the
character of the polarisation.

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