n particularly applied to
the settlements of Jews in foreign countries outside Palestine. These
were either voluntary, for purposes of trade and commerce, or the
results of conquest, such as the captivities of Assyria and Babylonia.
The word _diaspora_ (Gr. [Greek: diaspora]) is also used of these
scattered communities, but is usually confined to the dispersion among
the Hellenic and Roman peoples, or to the body of Christian Jews outside
Palestine (see JEWS).

DISPERSION, in OPTICS. When a beam of light which is not homogeneous in
character, i.e. which does not consist of simple vibrations of a
definite wave-length, undergoes refraction at the surface of any
transparent medium, the different colours corresponding to the different
wave-lengths become separated or _dispersed_. Thus, if a ray of white
light AO (fig. 1) enters obliquely into the surface of a block of glass
at O, it gives rise to the divergent system of rays ORV, varying
continuously in colour from red to violet, the red ray OR being least
refracted and the violet ray OV most so. The order of the successive
colours in all colourless transparent media is red, orange, yellow,
green, blue, indigo and violet. Dispersion is therefore due to the fact
that rays of different colours possess different refrangibilities.

[Illustration: FIG. 1.]

The simplest way of showing dispersion is to refract a narrow beam of
sunlight through a prism of glass or prismatic vessel containing water
or other clear liquid. As the light is twice refracted, the dispersion
is increased, and the rays, after transmission through the prism, form a
divergent system, which may be allowed to fall on a sheet of white
paper, forming the well-known solar spectrum. This method was employed
by Sir Isaac Newton, whose experiments constitute the earliest
systematic investigation of the phenomenon. Let O (fig. 2) represent a
small hole in the shutter of a darkened room, and OS a narrow beam of
sunlight which is allowed to fall on a white screen so as to form an
image of the sun at S. If now the prism P be interposed as in the
figure, the whole beam is not only refracted upward, but also spread out
into the spectrum RV, the horizontal breadth of the band of colours
being the same as that of the original image S. In an experiment similar
to that here represented, Newton made a small hole in the screen and
another small hole in a second screen placed behind the first. By
slightly turning the prism P,