1. _Light emanates or radiates from luminous bodies in a straight line._ This property is proved by the impossibility of seeing light through bent tubes, or small holes pierced in metallic plates placed one behind another, except the holes be placed in a straight line. If we endeavour to look at the sun or a candle through the bore of a bended pipe, we cannot perceive the object, nor any light proceeding from it, but through a straight pipe the object may be perceived. This is likewise evident from the form of the rays of light that penetrate a dark room, which proceed straight forward in lines proceeding from the luminous body; and from the form of the _shadows_ which bodies project, which are bounded by right lines passing from the luminous body, and meeting the lines which terminate the interposing body. This property may be demonstrated to the eye, by causing light to pass through small holes into a dark room filled with smoke or dust. It is to be understood, however, that in this case, the rays of light are considered as passing through the same medium; for when they pass from air into water, glass, or other media, they are bent at the point where they enter a different medium, as we shall afterwards have occasion to explain. 2. _Light moves with amazing velocity._ The ancients believed that it was propagated from the sun and other luminous bodies _instantaneously_; but the observations of modern astronomers have demonstrated that this is an erroneous hypothesis, and that light, like other projectiles, occupies a certain time in passing from one part of space to another. Its velocity, however, is prodigious, and exceeds that of any other body with which we are acquainted. It flies across the earth’s orbit--a space 190 millions of miles in extent, in the course of sixteen and a half minutes, which is at the rate of 192,000 miles every second, and more than a million of times swifter than a cannon ball flying with its greatest velocity. It appears from the discoveries of Dr. Bradley, respecting the aberration of the stars, that light flies from those bodies, with a velocity similar, if not exactly the same; so that the light of the sun, the planets, the stars, and every luminous body in the universe is propagated with _uniform_ velocity.[3] But, if the velocity of light be so very great, it may be asked, how does it not strike against all objects with a force equal to its velocity? If the finest sand were thrown against our bodies with the hundredth part of this velocity, each grain would pierce us as certainly as the sharpest and swiftest arrows from a bow. It is a principle in mechanics that the force with which all bodies strike, is in proportion to the size of these bodies, or the quantity of matter they contain, multiplied by the velocity with which they move. Therefore if the particles of light were not almost infinitely small, they would, of necessity prove destructive in the highest degree. If a