. Each of these clouds twists the plane of
polarisation 90 deg., causing the centre of the ring-system to change from
black to white, and the rings themselves to emit their complementary
colours. [Footnote: Sir John Herschel suggested to me that this
change of the polarisation from positive to negative may indicate a
change from polarisation by reflection to polarisation by refraction.
This thought repeatedly occurred to me while looking at the effects;
but it will require much following up before it emerges into
clearness.]

Almost all liquids have motes in them sufficiently numerous to
polarise sensibly the light, and very beautiful effects may be
obtained by simple artificial devices. When, for example, a cell of
distilled water is placed in front of the electric lamp, and a thin
slice of the beam is permitted to pass through it, scarcely any
polarised light is discharged, and scarcely any colour produced with a
plate of selenite. But if a bit of soap be agitated in the water above
the beam, the moment the infinitesimal particles reach the light the
liquid sends forth laterally almost perfectly polarised light; and if
the selenite be employed, vivid colours flash into existence. A still
more brilliant result is obtained with mastic dissolved in a great
excess of alcohol.

The selenite rings, in fact, constitute an extremely delicate test as
to the collective quantity of individually invisible particles in a
liquid. Commencing with distilled water, for example, a thick slice
of light is necessary to make the polarisation of its suspended
particles sensible. A much thinner slice suffices for common water;
while, with Bruecke's precipitated mastic, a slice too thin to produce
any sensible effect with most other liquids, suffices to bring out
vividly the selenite colours.

3. THE SKY OF THE ALPS.

The vision of an object always implies a differential action on the
retina of the observer. The object is distinguished from surrounding
space by its excess or defect of light in relation to that space. By
altering the illumination, either of the object itself or of its
environment, we alter the appearance of the object. Take the case of
clouds floating in the atmosphere with patches of blue between them.
Anything that changes the illumination of either alters the appearance
of both, that appearance depending, as stated, upon differential
action.

Now the light of the sky, being polarised, may, as the reader of