Dioptrics

Since, therefore, physicians In this context, physici refers to natural philosophers or scientists who study the physical world. say that a transparent substance, thinner than this air of ours, is spread above our heads, they cannot deny that the rays of visible objects are refracted when passing through the boundaries where the surfaces of the thick air and the thinner fire meet, wherever they pass through obliquely. Moreover, all rays except for one The ray coming from the zenith, directly overhead, is the only one that strikes the atmosphere perpendicularly and thus does not bend. pass obliquely to the observer’s location. Therefore, great refractions of rays would occur from every direction.

The force of this argument can be explained to the eye, as it were, by an experiment. Let the Sun shine against a wall. Let a censer thuribulum: a vessel used for burning incense or coals with burning coals be placed in between. If the air is calm, a certain stream of fiery substance will rise straight up from the censer, with no smoke mixed in; but if a slight breeze blows through, that stream will deflect a little to the side, yielding to the wind as it rises, yet bubbling with its own wavy motion. You will not perceive this stream of fire with your eyes, as it is completely colorless and transparent. But if you look at the opposite wall, you will see the shadows of things placed in the sun beyond the burning coal tremble—those shadows which are projected through this flow of fire.

Indeed, this trembling is a type of motion. Thus, the rays of the sun, which define the shadow, tremble because they are refracted as they pass through that fiery bubbling. They do so differently according to the changing surfaces of that fiery flow. From this inconsistent bending of the rays at the surface of that bubbling, there results an inconsistent landing of the bent or refracted rays upon the wall, and an inconsistent—that is, trembling—projection of the shadow. Therefore, it is established by this experiment that rays of light are sensibly refracted at the surface of a fiery substance, no matter how invisible it may be.

Consequently, no such fiery substance is spread out under the expanse of the heavens, hanging over our heads—neither a fluctuating one nor a calm one—because observers of the stars detect no trembling refraction or change of position in the stars, nor even a constant one that would be proportional to the shape of a sphere of fire; in short, they find no refraction other than that which occurs at the surface of the air.

This most solid argument, Pena again handles carelessly: