nd the results to spectra having
their origin in prisms instead of gratings.

10. _Diffraction when the Source of Light is not seen in Focus._--The
phenomena to be considered under this head are of less importance than
those investigated by Fraunhofer, and will be treated in less detail;
but in view of their historical interest and of the ease with which many
of the experiments may be tried, some account of their theory cannot be
omitted. One or two examples have already attracted our attention when
considering Fresnel's zones, viz. the shadow of a circular disk and of a
screen circularly perforated.

Fresnel commenced his researches with an examination of the fringes,
external and internal, which accompany the shadow of a narrow opaque
strip, such as a wire. As a source of light he used sunshine passing
through a very small hole perforated in a metal plate, or condensed by a
lens of short focus. In the absence of a heliostat the latter was the
more convenient. Following, unknown to himself, in the footsteps of
Young, he deduced the principle of interference from the circumstance
that the darkness of the interior bands requires the co-operation of
light from both sides of the obstacle. At first, too, he followed Young
in the view that the exterior bands are the result of interference
between the direct light and that reflected from the edge of the
obstacle, but he soon discovered that the character of the edge--e.g.
whether it was the cutting edge or the back of a razor--made no material
difference, and was thus led to the conclusion that the explanation of
these phenomena requires nothing more than the application of Huygens's
principle to the unobstructed parts of the wave. In observing the bands
he received them at first upon a screen of finely ground glass, upon
which a magnifying lens was focused; but it soon appeared that the
ground glass could be dispensed with, the diffraction pattern being
viewed in the same way as the image formed by the object-glass of a
telescope is viewed through the eye-piece. This simplification was
attended by a great saving of light, allowing measures to be taken such
as would otherwise have presented great difficulties.

In theoretical investigations these problems are usually treated as of
two dimensions only, everything being referred to the plane passing
through the luminous point and perpendicular to the diffracting edges,
supposed to be straight and parallel. In strict