De praecipius visus symptomatis eorumque causis Physica & Medica contemplatio

Hence that famous saying of Heraclitus about the essence of the animal spirit: auge xera sophotate a dry light is the wisest.

XXV.
The proper object of vision is light, then color, which exists as a certain diminished light, born from a mixture of the opaque and light, and for that reason more apt for vision. Since, just as a greater fire dazzles a lesser one, so external light blinds the intrinsic brightness of the eyes; the same light, tempered by some darker cloud, passes into a middle nature consistent with the sense.

XXVI.
Aristotle divides color into two primary species, but located at the extremes, namely White and Black. White is that in which there is the most light and the least opaque; akin to this is the Whitish, so called from its coruscating splendor. Black has little light and the most opaque.

XXVII.
There are five intermediate colors, distinguished by names: xanthos yellow, phoinikos red/crimson, halourges purple/sea-purple, prasinos green, and kyanos blue/azure. To the Latins: brown, red, purple, green, and blue. To each of these, many other species are subjected with such a variety of mixture that Plato not undeservedly called them ta chromata poikilata variegated colors, even threatening a penalty for curiously scrutinizing the proportion of such a mixture, which is known to God alone. Note (Greek letter Tau)

XXVIII.
Furthermore, the intermediate diaphanē transparent bodies, by which, once received, the phaseis appearances of colors are offered to the eye, are twofold. Some are aorista indefinite, such as Air and Water. The sky and the superior fire, insofar as they are transparent, are also referred here, which nevertheless differ from the former in that they illuminate both themselves and others by innate light and lead them into act.

XXIX.
Horismena defined or limited media are called solid bodies, composed of a pure nature, so that they are permeable to light, such as glass, etc. In these, the parts of water mixed with the purest earth exactly produce the to diaphanes the transparent. Whence even salt, often refined and purged of thicker dregs, congeals into a transparent crystal, as is evident in nitre, vitriol, alum, and others. Glass, too, a liquid of ashes, is of the same condition.