SECT. I. Of Light. CHA P. I. CONCERNING THE MOTION OF LIGHT. IN the former part of this Treatise our attention x has been employed in confidering fuch effects as arife from the motions and mutual actions of bodies, of magnitude fufficient to become the objects of our fenfes. It may be easily seen, that phenomena of this kind are but few compared with the very great number of operations which arise from the actions of bodies too minute to be determined but by deductions or inferences made from their effects. We have contemplated the great outline of the univerfe, and its vastness cannot but excite the astonishment of creatures who are deftined at present to occupy an exceedingly small part of it. As we proceed to examine that small part, we shall develope a scene of another kind, which, though expanded through a less portion of space, is equally immense and unlimited with regard to the field it affords for admiration and perpetual discovery. The fuccefs of every enquiry depends in no small y degree on the order employed in the several researches fearches to be made. It is obvious, that the probability of error is greater the more complicated the subject; whence the neceffity of first examining the moft fimple, and thence proceeding to more compounded objects, is evident. This principle leads us, in our confideration of the particular properties of various bodies, to attend first to those of light. z It is generally agreed, that light confifts of small bodies or particles, projected with great velocity in all directions, from the luminous or radiant body. No folid objections have been made to this hypothefis, which appears to be more fimple than any other, and is perfectly confiftent with all the phenomena yet obferved; and these are so many and fo various as to leave very little doubt of its truth. A The velocity of light was first determined by Monf. Romer, from obfervations on Jupiter's Moons (140, w) and the measure deduced from his obfervations was afterwards confirmed and eftablished by the discovery of the aberration of the fixed ftars. The principles on which this difcovery is founded may be familiarly explained as follows*. B Suppose a tube to be erected perpendicular to the horizon, at a time when it rains, the drops falling perpendicularly down, and suppose the diameter of the tube to be fuch as to admit but one drop at a time: then it is plain, that if a * The original account of this difcovery may be seen in a paper by its inventor Dr. Bradley, inferted in the Philofophical Tranfactions for the year 1728, No. 406. drop drop of water enter the orifice of the tube it will fall to the bottom without touching its fides. But if the tube, without altering its perpendicularity, be moved along in the direction of the horizon, any drop that enters will ftrike against one of its fides, and none will pafs clear through while the mo→ tion continues, unless the upper end of the tube be also inclined towards the part to which its motion is directed. Thus, if A B (fig. 70) represent the horizon, CD c the perpendicular tube, and G D the course of a drop of rain: then, if c D be moved towards A, while the drop is falling within the tube, it is evident that the inner furface of the tube, which is fituated towards B, will be carried against the drop, and prevent its arriving at the bottom without touching. But if the inclined tube EC be moved with a fimilar motion to that of the drop from E to D, in the time that the drop moves from c to D, the lower orifice of the tube and the drop will be found at o at the fame inftant; and the velocity of the drop will be expreffed by CD, and that of the tube by ED. The fame reafoning holds good, if inftead of D drops of rain we fuppofe particles of light, and a telescope instead of a tube. For to an obferver, who through the tube CD views the vaftly diftant object e, if the motion of light be instantaneous, or infinitely swift, no finite motion of c D, its pofition being unaltered, can prevent its being visible; fince, by the fuppofition, the light which enters at e will arrive at D before CD can have moved at all. But if light be propagated in time, and the obferver be carried by a motion fimilar, as to acceleration, to that of light, the tube must be inclined to the ray in an angle, whofe fine is to the fine of CED, or the angle the tube makes with the line of the obferver's motion, as the velocity of the obferver is to the velocity of light. For in the triangle DC E, the fides DE and DC, which exprefs thefe velocities, are as the fines of their oppofite angles. Hence if the angle of the inclination of the tube to the ray of light, together with the velocity and direction of the obferver's motion be known, the velocity of light may be determined. By this theory, which is established by a great number of observations on ftars of different magnitudes and fituations, it appears, that the small apparent motion the fixed ftars have about their real places, which is called their Aberration, arifes from the proportion which the velocity of the Earth's motion in her orbit bears to that of F light. This proportion is found to be as 10210 to 1: from whence it follows, that light moves or is propagated as far as from the Sun to the Earth Gin 8' 12". And it likewife appears, that the velo city of light is uniform, and the fame, whether original, as from the stars, or reflected, as from the fatellites of Jupiter (140, w). The velocity of light being known, an estimate might be made of the magnitude of its particles, if we were in poffeffion of good obferva |
