f copper
foil perforated by a fine needle hole. Observed through this the
structure of some wire gauze just disappeared at a distance from the
eye equal to 17 in., the gauze containing 46 meshes to the inch. On
the other hand, a single wire 0.034 in. in diameter remained fairly
visible up to a distance of 20 ft. The ratio between the limiting
angles subtended by the periodic structure of the gauze and the
diameter of the wire was (.022/.034) X (240/17) = 9.1. For further
information upon this subject reference may be made to _Phil. Mag._,
1896, 42, p. 167; _Journ. R. Micr. Soc._, 1903, p. 447.

6. _Coronas or Glories._--The results of the theory of the diffraction
patterns due to circular apertures admit of an interesting application
to _coronas_, such as are often seen encircling the sun and moon. They
are due to the interposition of small spherules of water, which act the
part of diffracting obstacles. In order to the formation of a
well-defined corona it is essential that the particles be exclusively,
or preponderatingly, of one size.

If the origin of light be treated as infinitely small, and be seen in
focus, whether with the naked eye or with the aid of a telescope, the
whole of the light in the absence of obstacles would be concentrated
in the immediate neighbourhood of the focus. At other parts of the
field the effect is the same, in accordance with the principle known
as Babinet's, whether the imaginary screen in front of the
object-glass is generally transparent but studded with a number of
opaque circular disks, or is generally opaque but perforated with
corresponding apertures. Since at these points the resultant due to
the whole aperture is zero, any two portions into which the whole may
be divided must give equal and opposite resultants. Consider now the
light diffracted in a direction many times more oblique than any with
which we should be concerned, were the whole aperture uninterrupted,
and take first the effect of a single small aperture. The light in the
proposed direction is that determined by the size of the small
aperture in accordance with the laws already investigated, and its
phase depends upon the position of the aperture. If we take a
direction such that the light (of given wave-length) from a single
aperture vanishes, the evanescence continues even when the whole
series of apertures is brought into contemplation. Hence, whatever
e