We give this designation to colours which are produced by certain material mediums: these mediums, however, have no colour themselves, and may be either transparent, semi-transparent yet transmitting light, or altogether opaque. The colours in question are thus produced in the eye through such external given causes, or are merely reflected to the eye when by whatever means they are already produced without us. Although we thus ascribe to them a certain objective character, their distinctive quality still consists in their being transient, and not to be arrested. 137. They are called by former investigators _colores apparentes, fluxi, fugitivi, phantastici, falsi, variantes_. They are also called _speciosi_ and _emphatici_, on account of their striking splendour. They are immediately connected with the physiological colours, and appear to have but little more reality: for, while in the production of the physiological colours the eye itself was chiefly efficient, and we could only perceive the phenomena thus evoked within ourselves, but not without us, we have now to consider the fact that colours are produced in the eye by means of colourless objects; that we thus too have a colourless surface before us which is acted upon as the retina itself is, and that we can perceive the appearance produced upon it without us. In such a process, however, every observation will convince us that we have to do with colours in a progressive and mutable, but not in a final or complete, state. 138. Hence, in directing our attention to these physical colours, we find it quite possible to place an objective phenomenon beside a subjective one, and often by means of the union of the two successfully to penetrate farther into the nature of the appearance. 139. Thus, in the observations by which we become acquainted with the physical colours, the eye is not to be considered as acting alone; nor is the light ever to be considered in immediate relation with the eye: but we direct our attention especially to the various effects produced by mediums, those mediums being themselves colourless. 140. 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. 141.