lse may happen, there must be a system of dark rings formed, the same
as from a single small aperture. In directions other than these it is
a more delicate question how the partial effects should be compounded.
If we make the extreme suppositions of an infinitely small source and
absolutely homogeneous light, there is no escape from the conclusion
that the light in a definite direction is arbitrary, that is,
dependent upon the chance distribution of apertures. If, however, as
in practice, the light be heterogeneous, the source of finite area,
the obstacles in motion, and the discrimination of different
directions imperfect, we are concerned merely with the mean brightness
found by varying the arbitrary phase-relations, and this is obtained
by simply multiplying the brightness due to a single aperture by the
number of apertures (n) (see INTERFERENCE OF LIGHT, S 4). The
diffraction pattern is therefore that due to a single aperture, merely
brightened n times.

In his experiments upon this subject Fraunhofer employed plates of
glass dusted over with lycopodium, or studded with small metallic
disks of uniform size; and he found that the diameters of the rings
were proportional to the length of the waves and inversely as the
diameter of the disks.

In another respect the observations of Fraunhofer appear at first
sight to be in disaccord with theory; for his measures of the
diameters of the red rings, visible when white light was employed,
correspond with the law applicable to dark rings, and not to the
different law applicable to the luminous maxima. Verdet has, however,
pointed out that the observation in this form is essentially different
from that in which homogeneous red light is employed, and that the
position of the red rings would correspond to the _absence_ of
blue-green light rather than to the greatest abundance of red light.
Verdet's own observations, conducted with great care, fully confirm
this view, and exhibit a complete agreement with theory.

By measurements of coronas it is possible to infer the size of the
particles to which they are due, an application of considerable
interest in the case of natural coronas--the general rule being the
larger the corona the smaller the water spherules. Young employed this
method not only to determine the diameters of cloud particles (e.g.
1/1000 in.), but also those of fibrous material, for which th