o the illuminating beam.
This continued to be the case as long as the cloud maintained a
decided blue colour, and even for some time after the blue had changed
to whitish blue. But, as the light continued to act, the cloud became
coarser and whiter, particularly at its centre, where it at length
ceased to discharge polarised light in the direction of the
perpendicular, while it continued to do so at both ends.

But the cloud which had thus ceased to polarise the light emitted
normally, showed vivid selenite colours when looked at obliquely,
proving that the direction of maximum polarisation changed with the
texture of the cloud. This point shall receive further illustration
subsequently.

A blue, equally rich and more durable, was obtained by employing the
nitrite-of-butyl vapour in a still more attenuated condition. The
instance here cited is representative. In all cases, and with all
substances, the cloud formed at the commencement, when the
precipitated particles are sufficiently fine, is _blue_, and it can be
made to display a colour rivalling that of the purest Italian sky. In
all cases, moreover, this fine blue cloud polarises _perfectly_ the beam
which illuminates it, the direction of polarisation enclosing an angle
of 90 deg. with the axis of the illuminating beam.

It is exceedingly interesting to observe both the perfection and the
decay of this polarisation. For ten or fifteen minutes after its
first appearance the light from a vividly illuminated actinic cloud,
looked at perpendicularly, is absolutely quenched by a Nicol's prism
with its longer diagonal vertical. But as the sky-blue is gradually
rendered impure by the growth of the particles--in other words, as
real clouds begin to be formed--the polarisation begins to decay, a
portion of the light passing through the prism in all its positions.
It is worthy of note, that for some time after the cessation of
perfect polarisation, the residual light which passes, when the Nicol
is in its position of minimum transmission, is of a gorgeous blue, the
whiter light of the cloud being extinguished. [Footnote: This shows
that particles too large to polarise the blue, polarise perfectly
light of lower refrangibility.] When the cloud texture has become
sufficiently coarse to approximate to that of ordinary clouds, the
rotation of the Nicol ceases to have any sensible effect on the
quantity of light discharged normally.

The perfection of the polarisation,