C. How Illuming is Generated by Light

18. And as to the third article [n.9], which makes the second article clear, one needs to note that illuming is generated by light according to a threefold root: direct ray, refracted ray, and reflected ray (according to Alhazen in his Optics 7.1 n.1).“Sight comprehends visibles in three ways, namely directly, by reflection from clean bodies, and by refraction on the other side of transparent bodies that differ in transparency from the transparency of the air.” 4.1 n.1 “The acquisition of seen objects is diversified in three ways, either directly, or by reflection on polished bodies, or by penetration, as in rare bodies whose rarity is not like the rarity of air...” ²⁷

19. A direct ray is one that flows from a luminous body in a medium of the same transparency in a direct line, and stops at the limit of the terminating or terminated body as long as the power of the luminous body lasts. A reflex ray is one that, when an opaque body intervenes (before the active power of the luminous body reaches its limit), flows in an opposed direction, not by choice, but naturally, because the natural agent whose active power is not completely used up in direct line acts as much as it can, and when it cannot act on a straight line it does so on an oblique one. A refracted ray is one that, when a medium of different transparency (but not completely opaque) intervenes, is continued in that medium though not in a straight line but there is an angle incident there; now when a more dense medium intervenes the ray is refracted toward the perpendicular, because from the fact that a perpendicular force is strongest in acting (hence it is not fractured) a position closer to a perpendicular ray is required for acting on a denser medium (such is the position of a ray refracted to the perpendicular); but when a less dense medium intervenes, the ray is refracted away from the perpendicular, for the opposite reason.The description here (whatever is said about the proposed reason) is correct. A ray of light coming from the air and falling on water at an angle is bent inwards, as it were, toward the perpendicular, and vice versa when coming from water into air. ²⁸

20. I say then that the illuming continued according to these three rays is generated by the light itself, and is also immediately a sensible species of the light.

21. Direct ray is plain. The proof about reflected ray is, first, that by this ray the thing is seen in itself and not in any species impressed by the reflecting mirror; second (see the conclusion below, nn.24-27). In the same way too is the thing itself seen by the refracted ray continuing itself, and not any species of the thing seen.

22. And just as these three primary illumings, diffused by multiplication, are immediately species of the light generating them, perhaps they are likewise as immediately generated by it, speaking of their immediacy to the cause, not excluding the order of the effects ordered in respect of the same cause.

23. But besides these three there is another, secondary illuming (which is called ‘accidental illuming), of the sort that is where a shadow is; for actual shadow is distinguished from darkness in this, that darkness is a privation of illuming both primary and secondary, but shadow is not a privation of secondary illuming. Now although this secondary illuming is sometimes diffused by reflection, though not from a polished body, for reflection from a polished body is the generation of a primary illuming because it is immediately a species of the light generating it; but not so this secondary illuming, which rather is generated by the primary illuming, so that if, per impossibile, reflection were to remain within the illuming of the impressing ray, it would generate this secondary illuming in a spherical, or semi-spherical, manner; but reflected illuming is not generated in this way by a primary ray of the sort that reflection from a polished body is.

So therefore all illuming is a species; but primary illuming is a species of the light generating it, and secondary illuming a species of primary illuming.

24. Hereby is the response clear to the arguments that could be made against this section [nn.20-23], that, since illuming is seen, it will not be a visible species.

Response:

Notwithstanding that an intended species is the intention and species of another visible thing, yet it can be visible in itself, as the Commentator says on Sense and Thing Sensed 2.437a23-24 [Averroes Paraphrase thereon ch.3], “when illuming is reflected by green plants, the walls appear colored.”

25. Likewise species are not visible save when they are stopped at opaque bodies, so that, when they are not in contact with the opaque, they cannot generate a species of themselves whereby they may be seen (as the species of color is not seen while it is in the medium); but when a species is in contact with the opaque, then it can be seen, as is plain about a ray passing through colored glass; the color appears on the wall when the ray is stopped at it and the wall is seen colored like the glass, and yet the colored ray was not visible in the medium; nor for this reason is the red that appears on the wall denied to be a species, for the eye focused on the wall would, if the red were put in its way, see the red glass or the redness of the glass.

26. Hence this proposition is false: ‘that which is the intention of the visible object is not visible’; but this one is true: ‘every intention is not only visible but is a reason for seeing’. Hence the form that illumines the moon is illuming (and not light, because it is not a body terminated in the moon), and yet it is sensible and is a species of light.

27. Likewise, from Aristotle at the beginning of Sense and Thing Sensed 2.437a23-24, “the gleam emitted from an eye in darkness is visible,” and yet it is a species of light only.

28. In the same way, illuming not terminated at an opaque body is not seen. The point is clear, first, because the solar rays do not appear at night, and yet they intercept between us and the sky at the tip of the cone of shadow cast by the earth; second, because someone in an opaque and dark prison, where a solar ray passed through opposite holes, would not see - so that, when the solar ray is not reflected anywhere on something opaque, he would not see the ray, unless the corpuscles are refracted.

29. Likewise, if an illumined medium were infinite and did not end at an opaque body, nothing would be seen when the sun was above the horizon, because there would be no end to the illuming.

30. Illuming then has the same condition as to being seen and not being seen as other principal species of colors have; and just as the principal species of colors, namely those that are continuied in rays, can generate other accidental secondary species not continued in rays, and can be seen through them (and yet there is no denying that the first species are intentions, because they are simply likenesses of the first generating colors); so it is in the issue at hand.

31. But that the whole ray is generated immediately by the light and not one part of the ray by another [n.20] is proved thus:

Because if it is posited that illuming in one part of the medium generates illuming from itself, then any point of illuming multiplies itself spherically (because every natural active principle acts spherically on the surrounding medium if it is equally disposed on all sides); and even if there were an opaque body in the way on one side of it, at any rate it multiplies itself hemi-spherically; therefore in the same way, if illuming were to be generative with respect to another illuming, then, from its not having an opaque body in the way on any side of it, it multiplies itself spherically; and thus, just as a principal illuming generates further another secondary, indirect illuming, so a secondary illuming would be generated from every part of a distant ray, which is contrary to the senses.

32. In addition, any luminous body can act at a distance in place (because if it could only act where it was in place it could only illumine something indivisible [sc. place is the surface of the containing body, and surface is an indivisible quantity]). But if it is present, in idea of agent, in some distant place, then, and by parity of reasoning, it is present in the whole medium, to as great a distance as its power is sufficient to reach; and if it is present to the whole and can cause illuming in the whole, then it does so cause it -because, even given that the illuming generated by it could cause another illuming, yet the present luminous body is of greater power than the illuming generated, and so it will precede the illuming in acting on a remote part.The Vatican editors remark that the proof in these two paragraphs, nn.31-32, was omitted or forgotten here and placed by Scotus after n.39 (the same sort of thing has apparently happened elsewhere). The Vatican editors have restored it to its place here. ²⁹