Treasury of Optics

...dark at night: when the air is dark, we see the stars; however, when the air between our sight and the stars is illuminated During the day, the atmosphere scatters sunlight, creating a bright background that prevents us from seeing the relatively faint light of the stars., the stars will be hidden from us. Similarly, if an observer at night looks at a place lit by the light of a fire, and the firelight is spread across the ground, and there are subtle visible things in that place, or objects containing fine details, and they are in some shadow (though not a deep one), and the fire is not positioned between those objects and the sight, the observer will then perceive those visible things and the fine details within them. But if he then moves from his position until the fire is between those objects and his sight, then those things will be hidden from him—both the fine objects themselves and the subtle details within them—and he will hardly perceive them at all when the fire is between his sight and the objects. Yet, if the fire is covered from his view, he will immediately perceive those visible things that were previously hidden; and if the cover between his sight and the fire is removed, those objects will be hidden from him once more. These situations signify, therefore, that strong lights shining upon the sight and upon the air between the eye and the thing seen prevent the sight from perceiving certain visible objects whose own lights are weak.

And again, when an observer looks at a polished body original: "corpus tersum," referring to a smooth, reflective surface like metal or glass. that has subtle engravings upon it—provided those engravings are not of a different color from the body itself—and the observer is in a place of moderate light: if that body is then placed opposite the sun or a wall lit with strong light, some light will be reflected from it to the sight. The observer will find the light appearing on the surface of the body, at the point from which the light is reflected, to be stronger and more sparkling. In this situation, if the observer gazes at that polished body, he will not see any of the engravings located in the place of the strong, sparkling light. However, if the observer tilts that body away from that position so that the reflection is directed to another place, away from his sight, and there is otherwise moderate light upon that body, then the observer will perceive the engravings upon it which he previously could not perceive during the reflection of light from the body to his sight. Similarly, when light is reflected from a polished page original: "pagina tersa." This likely refers to a smooth parchment or a metal plate used for writing or printing. containing subtle engravings toward the sight, the eye will neither distinguish nor verify those engravings until the light is no longer reflected to the eye from that page; if the surface of the page is tilted so that its position is changed and light is no longer reflected from it to the eye, the sight will then perceive and distinguish those engravings.

Again, when a weak fire original: "ignis debilis," perhaps a small flame or glowing coal. is in a weak light, it will appear and be perceived by the sight; but when it is in the light of the sun, the body in which the fire exists will appear dense and colored with a strong, sparkling color The flame itself becomes invisible or looks like a solid glowing object in the sun's overwhelming brightness.. Furthermore, if some white body of clear whiteness is near that body, and that white body is in shadow or weak light, the color of the [fire] body will appear upon it, just as we described above; but if that white body is moved until it is in the light of the sun, that color which was upon it will then be hidden. If it is returned to the shadow, that shining color which is within it will appear. That color will appear upon it even in strong light if it is shaded by a dense body; if it remains in its place until the light upon it is weakened, the color upon it will appear; and if the shading body is removed so that the light upon the white body grows strong, the color upon it will be hidden. Similarly, when we move a translucent body original: "corpus diaphanum." colored with a sparkling color toward a very strong fire, and we move a white cloth into the shadow of that body, the color of that translucent body will appear upon that cloth, as we stated before. Then, if we bring another fire near that cloth so that its light shines upon the cloth, the color that appeared upon the cloth will be hidden, and nothing will appear except the whiteness of the cloth itself; if we remove that fire, the second color will appear upon the cloth again.

Furthermore, certain marine animals have shells and carapaces, and when they are in a dark place where there is no light, those shells will appear like fire; yet if an observer looks at them in the light of day or a fire, he will perceive them, but will not see light or any fire in them. Similarly, when the animal called the glowworm original: "noctiluca," literally "night-light." Usually refers to fireflies or glowworms. flies by night, it appears like a lamp; but when an observer looks at it in the light of day or the light of a fire, the animal will appear without fire. All these situations which we have declared signify, therefore, that the strong lights of visible things sometimes hide things that are in certain visible objects, and that weak lights sometimes manifest certain things that are in certain visible objects.

And again, certain things are many times hidden from the sight, such as subtle engravings and writings, when they are in dark places or in weak lights; but if they are brought out to luminous places of strong light, or placed in the light of the sun, the things within them which were hidden in dark places and weak lights will appear. Similarly, the sight cannot perceive the details of subtle engravings in dark places and weak lights; but when they are brought out into strong lights, they are perceived by the sight. It is signified by this discussion, therefore, that strong lights manifest many visible things, and that weak lights hide many visible things.

THAT THE COLORS OF BODIES ARE VARIED TO THE SIGHT according to the diversity of lights shining upon them. Chapter 3.

Again, we find that the colors of dense bodies colored with sparkling colors—such as Lapis Lazuli original: "Lazuleis.", wine-reds, and sky-blues—when they are in dark places and weak lights, appear dull; but when they are in strong light, their colors appear sparkling and clear. The more the light upon them is increased, the more the clarity of the sparkling color upon them will be increased. If any of these bodies is in a dark place where there is only a very small amount of light, that body will appear dark, and the sight will not distinguish its color, and it will seem almost black; but when it is brought out to luminous places with strong light, its color will appear and be distinguished.