object we may take the minute light aperture in the tin plate of a camera obscura, as prepared for paroptical experiments.
425.
If we look through a tube, the farther end of which is contracted or variously indented, the same colours appear.
426.
The following phenomena appear to me to be more nearly allied to the paroptical appearances. If we hold up a needle near the eye, the point appears double. A particularly remarkable effect again is produced if we look towards a grey sky through the blades of knives prepared for paroptical experiments. We seem to look through a gauze; a multitude of threads appear to the eye; these are in fact only the reiterated images of the sharp edges, each of which is successively modified by the