closely connected with other appearances already known to us, that it is hardly possible to separate them.
391.
The paroptical colours then are so called because the light must pass by an outline or edge to produce them. They do not, however, always appear in this case; to produce the effect very particular conditions are necessary besides.
392.
It is also to be observed that in this instance again light does not act as an abstract diffusion (361), the sun shines towards an edge. The volume of light poured from the sun-image passes by the edge of a substance, and occasions shadows. Within these shadows we shall presently find colours appear.
393.
But, above all, we should make the experiments and observations that bear upon our present inquiry in