Treasury of Optics

obscure at night: therefore, when the air is dark, we see the stars: but when the air between our vision and the stars is illuminated, the stars will be hidden from us. And similarly, if a viewer at night looks in a place illuminated by the light of a fire: and the light of the fire is spread over the ground: and there are subtle visible things in that place, or visible things in which there are subtle matters: and they are in some shadow, but not a strong one: and the fire is not between those visible things and the vision: and the viewer then perceives those visible things, and the subtle matters that are in them: then he moves from his place, until the fire is between those visible things and his vision: then those visible things will be hidden from him, if they are subtle, or the subtle things that are in them, and he will hardly perceive them when the fire is between his vision and those visible things: and if the fire is covered from his vision: he will immediately perceive those visible things that were hidden from him: and if the covering between his vision and the fire is removed: those visible things will be hidden from him again. These arrangements therefore signify that strong lights rising upon the vision and upon the air between the eye and the seen object prevent the vision from perceiving certain visible things whose lights are weak. And again, when a viewer gazes upon a polished body: and there are subtle carvings in that body: and those carvings are not of different colors from the color of the body: and the viewer is in a place of moderate light: then if that body is placed opposite the sun or a wall illuminated by strong light: some light will be reflected from it to the vision: and the viewer will find the light appearing on the surface of the body, and in the place from which the light is reflected, to be stronger and more sparkling: and in this arrangement, if the viewer gazes at that polished body: he will not see any of the carvings that are in the place of strong and sparkling light: then if the viewer moves that body from that place, so that the reflection occurs to another place, outside the place of his vision, and there is furthermore moderate light upon that body: then the viewer will perceive the carvings that are in it, which he did not perceive before in the reflection of light from the body to his vision. And similarly, when light is reflected from a polished page, in which there are subtle carvings, to the vision: the vision will not distinguish those carvings nor verify them, until the light reflected to the vision from that page is removed: and if the surface of the page is tilted, so that its position is changed, and the light is not reflected from it to the vision: the vision will then perceive those carvings and distinguish them. And again, when a weak fire is in weak light: it will appear and be perceived by the vision: and when it is in the light of the sun: the body in which it is will appear dense, colored with a strong, sparkling color: and then if there is near that body some white body of clear whiteness: and that body is in shadow and weak light: the color of that body will appear upon it, as we described above: then if that white body is moved until it is in the light of the sun, that color which is in it will be hidden: and if it is brought back to the shadow: that shining color which is in it will appear: and that color will appear upon it in strong light, and upon the hiding of the color that is on it, if it is shaded by a dense body: and if it remains in its place until the light that is upon it is weakened: the color that is upon it will appear: and if the shading body is removed, until the light upon the white body becomes vigorous: the color that is upon it will be hidden. And similarly, when we move a diaphanum transparent/translucent body colored with a sparkling color toward a vehemently strong fire: and we move a white cloth to the shadow of that body: the color of that transparent body will appear upon that cloth, as we described before: then if we move another fire toward that cloth, so that its light rises upon that cloth: the color that appeared upon the cloth will be hidden, and only the whiteness of the cloth will appear: and if we remove that fire, the second color will appear upon the cloth. And again, certain marine animals have shells and coverings, and when they are in a dark place where there is no light, those shells will appear as if they were fire: and if a viewer looks at them in the light of day or fire, he will perceive them, and will not see in them light or any fire. And similarly, when the animal that is called the noctiluca glow-worm/bioluminescent creature flies at night, it appears as if it were a lamp, and when a viewer looks at it in the light of day or in the light of fire, the animal will appear without fire. Therefore, all these arrangements which we have explained signify that the strong lights of visible things sometimes conceal things that are in certain visible things: and that weak lights sometimes manifest certain things that are in certain visible things. And again, certain things that are invisible, such as subtle carvings and writings, are often hidden from the vision when they are in dark places or in weak lights: and if they are taken out to luminous places of strong light, or placed in the light of the sun: the things that are in them, which were hidden in dark places and weak lights, will appear. And similarly, the vision cannot perceive the understandings of subtle carvings in dark places and weak lights: and when they are taken out to strong lights, they are perceived by the vision. It is signified therefore by this discussion that strong lights manifest many visible things, and that weak lights conceal many visible things.

THAT THE COLORS OF BODIES ARE DIVERSIFIED TO THE VISION ACCORDING TO THE DIVERSITY OF LIGHTS FALLING UPON THEM. Chap. 3.

3. Color is varied according to the quality of light.

And again we find that the colors of dense bodies colored with sparkling colors, such as azure, wine-colored, and celestial: when they are in dark places and weak lights, they appear turbid, and when they are in strong light, their colors appear sparkling and clear: and the more the light is increased upon them, the more the clarity of the sparkling color will be increased upon them. And if one of these bodies is in a dark place: and there is in it only a very small light: that body will appear dark, and the vision will not distinguish its color, and it will be seen as if it were black: and when it is taken out to luminous places with strong light: its color will appear, and will be distinguished from...