[XXXII.]
PAROPTICAL COLOURS.
The paroptical colours have been hitherto called peri-optical, because a peculiar effect of light was supposed to take place as it were round the object, and was ascribed to a certain flexibility of the light to and from the object.
These colours again may be divided into subjective and objective, because they appear partly without us, as it were, painted on surfaces, and partly within us, immediately on the retina. In this chapter we shall find it more to our purpose to take the objective cases first, since the subjective are so closely connected with other appearances already known to us, that it is hardly possible to separate them.
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.