their transparency, but acquire a certain original light. ARTICLE IX. THE AGREEMENTS AND DISAGREEMENTS OF LIGHT. THE agreements and disagreements of light must be enquired into. Light has a principal correspondence and affinity with three things, as to its generation, viz. heat, subtilty, and motion; the conjunction and separation whereof, in respect of light, must therefore be examined, together with their degrees. The flame of spirit of wine, or the ignis fatuus, is much milder in heat, but stronger in light than ignited iron. Glowworms, the dewy vapour raised by the dashing of salt water, and many other things above mentioned, yield light, though they are not warm to the touch. Ignited metals are not subtile bodies, yet they have a burning heat. On the contrary, air is one of the subtilest bodies, yet has no light. Again, winds, though very rapid in their motion, afford no light. Contrariwise, ignited metals have but a dull and languid motion, yet vibrate light. But there is nothing so nearly related to light, not indeed with regard to its generation, but only to its passage, as sound, and therefore their agreements and disagreements are to be diligently sought. Some of their agreements are these. 1. They both diffuse themselves in a sphere. 2. They both move to very great distances, but light the swiftest, as appears in the discharging of cannon, where the light is first perceived before the sound, and the flame follows after. 3. They have both very subtile differences,; sounds in the articulation of words, and light in all the images of visible things. 4. They both produce or generate little, except in senses and spirits of animals. 5. They both are easily generated, and soon vanish. 6. Light is drowned by a greater light, and sound by a greater sound, &c. Some of their differences are these. 1. Light moves swifter than sound. 2. Light moves farther than sound. 3. Light moves only in a straight line, but sound obliquely and every other way for when an object is perceived in the shade of an obstacle, it is not because the light penetrates the obstacle, but only illuminates the air round about, whence the air behind the obstacle is also somewhat illuminated. But a sound begun on one side of a wall is heard without much diminution on the other. And again, sound is heard from withinside a solid body, as in the eagle-stone, or from bodies struck under water; but light is not at all perceived in a transparent body that is every way obstructed or surrounded. 4. Lastly, all sound is generated in motion, and a manifest stroke of the sounding body, which in light is otherwise. But for the disagreements of light, there have been none hitherto observed, unless privations may be called disagreements. And it should seem that sluggishness in the parts of bodies is the greatest enemy to light, for scarce any thing is luminous that is not in its own nature remarkably moveable, or easily excited either by heat, motion, or vital spirit. But this enquiry should be further prosecuted. And we always mean, not only that other new instances should be diligently sought out, in conformity with those few which we only produce as a specimen, but likewise that new articles and tables of enquiry should be set down, added, and drawn up as the nature of the subject directs or requires.* *This subject of light is prosecuted to a considerable length by Dr. Hook, in his Lectures of Light; by Mr. Boyle, in his Experiments and Observations upon Colours; M. Huygens de la Lumiere; and since by Sir Isaac Newton, in his Treatise of the Reflections, Refractions, Inflections, and Colours of Light. |