corners, and their colours; just as in the above-mentioned appearance with the round opening.
408.
We have now subdued a parallactic light by causing it to shine through small apertures, but we have not taken from it its parallactic character; so that it can produce double shadows of bodies, although with diminished power. These double shadows which we have hitherto been describing, follow each other in light and dark, coloured and colourless circles, and produce repeated, nay, almost innumerable halos. These effects have been often represented in drawings and engravings. By placing needles, hairs, and other small bodies, in the subdued light, the numerous halo-like double shadows may be increased; thus observed, they have been ascribed to an alternating flexile action of the light, and the same assumption has been employed to explain the obliteration of the central shadow, and the appearance of a light in the place of the dark.
409.
For ourselves, we maintain that these again are parallactic double shadows, which appear edged with coloured borders and halos.
410.
After having seen and investigated the foregoing phenomena, we can proceed to the experiments with knife-blades,40 exhibiting effects which may be referred to the contact and parallactic mutual intersection of the half-shadows and halos already familiar to us.
411.
Lastly, the observer may follow out the experiments with hairs, needles, and wires, in the half-light produced as before described by the sun, as well as in that derived from the blue sky, and indicated on the white paper. He will thus make himself still better