1. The position of every band of light
and shade in such figures has been calculated from theory by Fresnel,
Fraunhofer, Herschel, Schwerd, and others, and completely verified by
experiment. Your eyes could not tell you with greater certainty of the
existence of these bands than the theoretic calculation.

[Illustration: Fig. 20.]

The street-lamps at night, looked at through the meshes of a
handkerchief, show diffraction phenomena. The diffraction effects
obtained in looking through a bird's feathers are, as shown by
Schwerd, very brilliant. The iridescence of certain Alpine clouds is
also an effect of diffraction which may be imitated by the
spores of Lycopodium. When shaken over a glass plate these spores
cause a point of light, looked at through the dusted plate, to be
surrounded by coloured circles, which rise to actual splendour when
the light becomes intense. Shaken in the air the spores produce the
same effect. The diffraction phenomena obtained during the artificial
precipitation of clouds from the vapours of various liquids in an
intensely illuminated tube are, as I have elsewhere shewn, exceedingly
fine.

[Illustration: Fig. 21.]

One of the most interesting cases of diffraction by small particles
that ever came before me was that of an artist whose vision was
disturbed by vividly coloured circles. He was in great dread of losing
his sight; assigning as a cause of his increased fear that the circles
were becoming larger and the colours more vivid. I ascribed the
colours to minute particles in the humours of the eye, and ventured to
encourage him by the assurance that the increase of size and vividness
on the part of the circles indicated that the diffracting particles
were becoming _smaller_, and that they might finally be altogether
absorbed. The prediction was verified. It is needless to say one word
on the necessity of optical knowledge in the case of the practical
oculist.

Without breaking ground on the chromatic phenomena presented by
crystals, two other sources of colour may be mentioned here. By
interference in the earth's atmosphere, the light of a star, as shown
by Arago, is self-extinguished, the twinkling of the star and the
changes of colour which it undergoes being due to this cause. Looking
at such a star through an opera-glass, and shaking the glass so as to
cause the image of the star to pass rapidly over the retina, you
produce a row of coloured beads, the spaces between which corresp