Tratado 29 (IV, 5, 4) – MUTUAL RELATION OF THE EYE’S LIGHT AND THE OBJECTIVE LIGHT (Guthrie)

MUTUAL RELATION OF THE EYE’S LIGHT AND THE OBJECTIVE LIGHT.

4. What is the mutual relation between the light that emanates from the eye, and the light which is exterior to the eye, and which extends between the eye and the object? Light has no need of air as a medium, unless indeed somebody should undertake to say that there is no light without air, in which case air would be a medium only accidentally. Light itself, however, is an unaffected medium, for there is no necessity here for an affection, but only for a medium; consequently, if light be not a body, there is no need of a body (to act as medium). It might be objected that sight has no need either of a foreign light nor of a medium to see near by, but has need of them for vision at a distance. Later we shall consider whether or not light without air be possible. Now let us consider the first point.

INTERMEDIARY LIGHT IS UNNECESSARY. PARTLY BEING AN OBSTACLE.

If the light which is contiguous to the eye should become animated, and if the soul should, so to speak, interpenetrate it, uniting with it as she unites with the interior light, there would be no need of intermediary light for the perception of the visible object. Sight resembles touch; it operates in light by somehow transferring itself to the object, without the medium experiencing any affection. Now consider: does the sight transfer itself to the visible object because of the existence of an interval between them, or because of the existence of some body in the interval? In the latter case, vision would occur by removing this obstacle. If, on the other hand, it be because of the existence of a mere interval, then the nature of the visible object must seem inert and entirely inactive. This is however impossible; not only does touch announce and experience the neighboring object but, by the affection it experiences, it proclaims the differences of the tangible object, and even perceives it from a distance, if nothing oppose it; for we perceive the fire at the same time as the air that surrounds us, and before this air has been heated by the fire. A solid body heats better than does the air; and consequently it receives heat through the air, rather than by the intermediation of air. If then the visible object have the power to act, and if the organ have the power of experiencing (or suffering), why should sight need any intermediary (besides light) to exert its power? This would really be needing an obstacle! When the light of the sun reaches us, it does not light up the air before lighting us, but lights both simultaneously; even before it has reached the eye, while it is still elsewhere, we have already seen, just as if the air was not affected at all; that is the case, probably, because the medium has undergone no modification, and because light has not yet presented itself to our view. Under this hypothesis (which asserts that the air receives and transmits an affection) it would be difficult to explain why during the night we see the stars and, in general, any kind of fire.

NOT EVEN THE LIGHT OF THE EYE IS TO BE CONSIDERED AS MEDIUM.

On the hypothesis that the soul remains within herself, while making use of the light (emanated from the eye) as a rod to reach the visible object, a very sharp perception would be caused by the resistance experienced by the light in its tension and sense-color. In so far as it is color, the light itself would possess the property of reflecting light. In this case, the contact would take place by a medium. But already before this the light has reached the object without any medium; so that the later contact operated by a medium would produce cognition by a sort of memory or reasoning — which is not the case.

THE OBJECTIVE LIGHT DOES NOT TRANSMIT THE IMAGE BY RELAYS.

The hypothesis that the light contiguous to the visible object is affected, and transmits this affection by relays from point to point into the eye, is essentially identical with that theory which supposes that the medium must be preliminarily modified by the visible object; a hypothesis that has already been discussed above.