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 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.