but a luminous image that we have to consider.
364.
In proceeding onwards to the paroptrical class, the reader, if duly acquainted with the foregoing facts, will be pleased to find himself once more in the region of circumscribed forms. The shadows of bodies, especially, as secondary images, so exactly accompanying the object, will serve greatly to elucidate analogous appearances.
365.
We will not, however, anticipate these statements, but proceed as heretofore in what we consider the regular course.
XXXI.
CATOPTRICAL COLOURS.
366.
Catoptrical colours are such as appear in consequence of a mirror-like reflection. We assume, in the first place, that the light itself as well as the surface from which it is reflected, is perfectly colourless. In this sense the appearances in question come under the head of