Light under these circumstances may be affected by three conditions. First, when it flashes back from the surface of a medium; in considering which catoptrical experiments invite our attention. Secondly, when it passes by the edge of a medium: the phenomena thus produced were formerly called perioptical; we prefer the term paroptical. Thirdly, when it passes through either a merely light-transmitting or an actually transparent body; thus constituting a class of appearances on which dioptrical experiments are founded. We have called a fourth class of physical colours epoptical, as the phenomena exhibit themselves on the colourless surface of bodies under various conditions, without previous or actual dye (βαφή).—[Note K].
In examining these categories with reference to our three leading divisions, according to which we consider the phenomena of colours in a physiological, physical, or chemical view, we find that the catoptrical colours are closely connected with the physiological; the paroptical are already somewhat more distinct and independent; the dioptrical exhibit themselves as entirely and strictly physical, and as having a decidedly objective character; the epoptical, although still only apparent, may be considered as the transition to the chemical colours.
If we were desirous of prosecuting our investigation strictly in the order of nature, we ought to proceed according to the classification which has just been made; but in didactic treatises it is not of so much consequence to connect as to duly distinguish the various divisions of a subject, in order that at last, when every single class and case has been presented to the mind, the whole may be embraced in one comprehensive view. We therefore turn our attention forthwith to the dioptrical class, in order at once to give the reader the full impression of the physical colours, and to exhibit their characteristics the more strikingly.