neighbourhood
of a heated body. At a moment when the eye, or object-glass of a
telescope, occupies a dark position, the star vanishes. A fraction of
a second later the aperture occupies a bright place, and the star
reappears. According to this view the chromatic effects depend
entirely upon atmospheric dispersion.

[10] In experiment a line of light is sometimes substituted for a
point in order to increase the illumination. The various parts of the
line are here _independent_ sources, and should be treated
accordingly. To assume a cylindrical form of primary wave would be
justifiable only when there is synchronism among the secondary waves
issuing from the various centres.

[11] H. Necker (_Phil. Mag._, November 1832); Fox Talbot (_Phil.
Mag._, June 1833). "When the sun is about to emerge ... every branch
and leaf is lighted up with a silvery lustre of indescribable
beauty.... The birds, as Mr Necker very truly describes, appear like
flying brilliant sparks." Talbot ascribes the appearance to
diffraction; and he recommends the use of a telescope.

DIFFUSION (from the Lat. _diffundere; dis-_, asunder, and _fundere_, to
pour out), in general, a spreading out, scattering or circulation; in
physics the term is applied to a special phenomenon, treated below.

1. _General Description._--When two different substances are placed in
contact with each other they sometimes remain separate, but in many
cases a gradual mixing takes place. In the case where both the
substances are gases the process of mixing continues until the result is
a uniform mixture. In other cases the proportions in which two different
substances can mix lie between certain fixed limits, but the mixture is
distinguished from a chemical compound by the fact that between these
limits the composition of the mixture is capable of continuous
variation, while in chemical compounds, the proportions of the different
constituents can only have a discrete series of numerical values, each
different ratio representing a different compound. If we take, for
example, air and water in the presence of each other, air will become
dissolved in the water, and water will evaporate into the air, and the
proportions of either constituent absorbed by the other will vary
continuously. But a limit will come when the air will absorb no more
water, and the water will absorb no more air, and throughout the change
a