of violet light must therefore fall nearer to the centre than the
maxima and minima of red light. The maxima and minima of the other
colours fall between these extremes. In this simple way the undulatory
theory completely accounts for the extraordinary appearance above
referred to.

When a slit and telescope are used, instead of the slit and naked eye,
the effects are magnified and rendered more brilliant. Looking,
moreover, through a properly adjusted telescope with a small circular
aperture in front of it, at a distant point of light, the point is
seen encircled by a series of coloured bands. If monochromatic light
be used, these bands are simply bright and dark, but with white light
the circles display iris-colours. If a slit be shortened so as to form
a square aperture, we have two series of spectra at right angles to
each other. The effects, indeed, are capable of endless variation by
varying the size, shape, and number of the apertures through which the
point of light is observed. Through two square apertures, with their
corners touching each other as at A, Schwerd observed the appearance
shown in fig. 20. Adding two others to them, as at B, he observed the
appearance represented in fig. 21. 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