e shall find that the same law holds good as before,
but that the proportion which passes is invariably greater with the red
than the blue. The question then presents itself: Is there any connection
between the amounts of the red and the blue which pass?

Lord Rayleigh, some years ago, made a theoretical investigation of the
subject. But, as far as I am aware, no definite experimental proof of the
truth of the theory was made till it was tested last year by General
Festing and myself. His law was that for any ray, and through the same
thickness, the light transmitted varied inversely as the fourth power of
the wave length. The wave length 6,000 lies in the red, and the wave
length 4,000 in the violet. Now 6,000 is to 4,000 as 3 to 2, and the
fourth powers of these wave lengths are as 81 to 16, or as about 5 to 1.
If, then, the four inches of our turbid medium allowed three quarters of
this particular red ray to be transmitted, they would only allow (3/4)^{5},
or rather less than one fourth, of the blue ray to pass.

Now, this law is not like the law of absorption for ordinary absorbing
media, such as colored glass for instance, because here we have an
increased loss of light running from the red to the blue, and it matters
not how the medium is made turbid, whether by varnish, suspended sulphur,
or what not. It holds in every case, so long as the particles which make
the medium turbid are small enough. And please to recollect that it
matters not in the least whether the medium which is rendered turbid is
solid, liquid, or air. Sulphur is yellow in mass, and mastic varnish is
nearly white, while tobacco smoke when condensed is black, and very
minute particles of water are colorless; it matters not what the color
is, the loss of light is _always_ the same. The result is simply due to
the scattering of light by fine particles, such particles being small in
dimensions compared with a wave of light. Now, in this trough is
suspended 1/1000 of a cubic inch of mastic varnish, and the water in it
measures about 100 cubic inches, or is 100,000 times more in bulk than
the varnish. Under a microscope of ordinary power it is impossible to
distinguish any particles of varnish; it looks like a homogeneous fluid,
though we know that mastic will not dissolve in water.

Now a wave length in the red is about 1/40000 of an inch, and a little
calculation will show that these particles are well within the necessary
limits. Prof. Tyndall has