TOPICS OF INQUIRY, CONCERNING LIGHT AND THE MATTER OF LIGHT. I. Presence Tables. WE have first to note which are the substances, of whatever kind, that generate light; as stars, fiery meteors, flame, wood, metals, and other burning bodies, sugar in scraping or breaking it, the glowworm, the dews of salt water when it is agitated or scattered, the eyes of certain animals, some sorts of rotten wood, large quantities of snow ; perhaps the air itself may possess a weak light adapted to the vision of the animals which see by night; iron and tin, when put into aqua fortis to be dissolved, boil, and without any fire produce intense heat, but whether or not they give out any light demands inquiry ; the oil of lamps sparkles in very cold weather; a kind of faint light is sometimes observed in a clear night around a horse that is sweating ; around the hair of certain persons, there is seen, though rarely, also a faint light, like a lambent flamule, as occured to Lucius Marcius in Spain ; there was lately found an apron of a certain woman which was said to shine, yet only when rubbed ; but it had been dyed in green, of which dye alum is an ingredient, and it rustled somewhat when shining. Whether alum shines or not when scraped or broken is matter of inquiry; but, I suppose, it requires more violent breaking, because it is firmer than sugar. In like manner, some stockings shine whilst you are pulling them off, whether from sweat or the dye of alum. II. Msence Tables. We must also observe which are the substances that give no light, yet have much similitude to such as do produce it. Boiling water does not give light; air though unusually heated does not give light; mirrors and diamonds, which so strikingly reflect light, give no light of their own. In this kind of instances we have also to con sider diligently the instances migratory, namely, when light, as if transient, is present, and when absent. A burning coal gives light, but loses it instantly when strongly compressed ; the crystal line humour of the glowworm, after the worm s death, even when broken and divided into parts, retains light for a short time, which, however, soon flftei fades away III. Table of Degrees. We must remark which sorts of light are more intense and vibrating, which less : the flame of wood produces a strong light; the flame of spirit of wine, a weaker; the flame of coals when fully kindled, a very dim and scarcely visible light. IV. Colours of Light. We have to consider the colours of light, what they are, what not ; some stars are white, others glittering, some red, some lead-coloured ; the common sorts of flame are generally croceous, and among these the coruscations from the sky, and the sparks from flint, tend most to whiteness ; the flame of sulphur is ceruleous and beautiful ; but in some substances are purple flames. No green flames are observed : what most inclines to greenness, is that of the glowworm. Neither are there any crimson flames discovered : heated iron is red, but if heated somewhat more intensely, it becomes as it were white. V. Reflections of Light. We have to observe what bodies reflect light : as mirrors, water, polished metals, the moon, precious stones. All liquid bodies and such as have very equal smooth surfaces are somewhat bright; but brightness is a certain small degree of light. We have to remark attentively, whether or not the light of one lucid body can be reflected by another lucid body ; as if you took heated iron and opposed it to the sun s rays. For the reflections of light are reflected on, yet b%coming gradually feebler, from mirror to mirror. VI. Multiplication of Light. The multiplication of light must next be con sidered : as by mirrors, perspectives, and the like, by which light may be sharpened and thrown to a distance, or also rendered subtler and softer for distinguishing visible objects; as you may see among painters, who use a phial filled with water beside their candle. Whether all bodies of any considerable size do not reflect light, must also be considered. For light, as may be believed, either goes through or is reflected : from which cause the moon, thougn
