409 TOPICS OF INQUIRY RESPECTING LIGHT AND LUMINOUS MATTER. I. The Table of Presence. OBSERVE first, all bodies of every kind which generate light; as stars, fiery meteors, flame, wood, metals, and other bodies ignited, sugar in scraping and breaking, the glowworm, spray of salt-water beaten and thrown about, the eyes of some animals, some kinds of rotten wood, a great mass of snow. The air itself may perhaps have a feeble light suited to the eyes of those animals which see at night. Iron and tin when put into aqua-fortis for solution boil up, and without any fire conceive a strong heat; but whether they emit any light is a point for inquiry. The oil of lamps sparkles in hard frosts; on a clear night a feeble light is sometimes visible about a sweating horse; and sometimes likewise, though seldom, about men's hair, in the shape of a small lambent flame; as happened to Lucius Marcius in Spain. A woman's stomacher was lately observed to shine, but only on being rubbed; this however had been dyed green, a dye in which alum is an ingredient, and it slightly crackled while it glittered. Inquire whether alum glitters on being scraped or broken; but I suppose it requires a stronger fracture than sugar, as being a more stubborn body. Some stockings have been observed to shine on being pulled off, either from sweat or alum dye. Other instances. II. The Table of Absence in the next Degree. Observe likewise what those bodies are which do not emit light, and yet have a great resemblance to those which do. Boiling water gives no light; neither does air though violently 1 Livy, xxv. 39. heated. Mirrors and diamonds, which reflect light so wonderfully, give none of their own. Other instances. Observe likewise accurately in this kind of instances respecting those that are migratory, that is, where light is present and absent, as it were in passing. An ignited coal gives light, but if it be strongly compressed it at once loses it. The crystalline moisture of the glowworm, at the death of the worm, though broken and divided into parts, retains its light for a short time; but this soon dies away. Other instances. III. The Table of Degrees. Observe the different intensities and vibrations of different kinds of light. The flame of wood emits a strong light; the flame of spirit of wine a weaker; the flame of coals thoroughly ignited one very dusky and hardly visible. Other instances. IV. Colours of Light. Observe concerning the colours of light, what kinds there are, and what not. Some of the stars are white, some bright, some reddish, and some lead-coloured. Common flames are generally saffron-coloured, and among them celestial coruscations and the flames of gunpowder are most inclined to whiteness. The flame of sulphur is a beautiful blue. Some bodies have purple flames. No green flames are yet discovered; the most inclined thereto is the light of the glowworn. Neither are there scarlet flames. Ignited iron is reddish, and when more intensely ignited, whitish. Other instances. V. Reflections of Light, Observe what bodies reflect light; as mirrors, waters, polished metals, the moon, and precious stones. All liquid bodies and such as have a very smooth and polished surface have some brightness; but brightness is a small degree of luminosity. Observe carefully whether the light of one lucid body can be reflected by another; as if ignited iron be taken and exposed to the sun's rays. For the reflections of light are reflected again from mirror to mirror, though they become gradually fainter and weaker. Other instances. |