I should
certainly have made without suggestion, was, as a matter of fact,
first definitely suggested by a remark addressed to me in a letter by
Professor Stokes.

All the phenomena of colour and of polarization observable in the case
of skylight are manifested by those actinic clouds; and they exhibit
additional phenomena which it would be neither convenient to pursue,
nor perhaps possible to detect, in the actual firmament. They enable
us, for example, to follow the polarization from its first appearance
on the barely visible blue to its final extinction in the coarser
cloud. These changes, as far as it is now necessary to refer to them,
may be thus summed up:--

1. The actinic cloud, as long as it continues blue, discharges
polarized light in all directions, but the direction of maximum
polarization, like that of skylight, is at right angles to the
direction of the illuminating beam.

2. As long as the cloud remains distinctly blue, the light discharged
from it at right angles to the illuminating beam is _perfectly_
polarized. It may be utterly quenched by a Nicol prism, the cloud from
which it issues being caused to disappear. Any deviation from the
perpendicular enables a portion of the light to get through the prism.

3. The direction of vibration of the polarized light is at right
angles to the illuminating beam. Hence a plate of tourmaline, with its
axis parallel to the beam, stops the light, and with the axis
perpendicular to the beam transmits the light.

4. A plate of selenite placed between the Nicol and the actinic cloud
shows the colours of polarized light; in fact, the cloud itself plays
the part of a polarizing Nicol.

5. The particles of the blue cloud are immeasurably small, but they
increase gradually in size, and at a certain period of their growth
cease to discharge perfectly polarized light. For some time afterwards
the light that reaches the eye, through the Nicol in its position of
least transmission, is of a magnificent blue, far exceeding in depth
and purity that of the purest sky; thus the waves that first feel the
influence of size, at both limits of the polarization, are the
shortest waves of the spectrum. These are the first to accept
polarization, and they are the first to escape from it.

LECTURE V.

RANGE OF VISION NOT COMMENSURATE WITH RANGE OF RADIATION
THE ULTRA-VIOLET BAYS
FLUORESCENCE
THE RENDERING OF INVISIBLE RAYS VISIBLE
VISION NOT THE ONLY SENSE APPE