The power of gravity tends down wards; but the pressure of water arising from it tends every way with an equable force, and is propagated, with equal ease and equal strength, in curves as in straight lines. Waves, on the surface of the wa ter, gilding by the extremes of any -very large obstacle, inflate and dilate them selves, still diffusing gradually into the quiescent water beyond that obstacle. The waves, pulses, or vibrations of the air, wherein sound consists, are manifest ly inflected, though not so considerably as the waves of water ; and sounds am propagated with equal ease through crooked tubes and through straight lines ; but light was never known to move iu any curve, nor to inflect itself ad um. brain." It must he acknowledged, however, that many philosophers, both English and foreigners, have recurred to the opinion, that light consists of vibrations propagat ed from the luminous body, through a subtle ethereal medium.
Dr. Franklin, in a letter dated April 23, 1752, expresses his dissatisfaction with the doctrine, that light consists of parti cles of matter continually driven off from the sun's surface, with so enormous a swiftness. "Must not," says he, "the smallest portion conceivable have, with such a motion, a force exceeding that of a twenty-four pounder discharged from a cannon ? Must not the sun diminish ex ceedingly by such a waste of matter ; and the planets, instead of drawing nearer to him, as some have feared, recede to great er distances, through the lessened attrac tion ? yet these particles, with this amaz ing motion, will not drive before them, or remove, the least and slightest dust they meet with, and the sun appears to continue of his ancient dimensions, and his attendants move in their ancient or bits." lie therefore conjectures, that all the phenomena of light may be more properly solved, by supposing all space filled with a subtle elastic 'fluid, which is not visible when at rest, but which, by its vibrations, affects the fine sense in the eye, as those of the air affect the grosser organs of the ear ; and even that different degrees of the vibration of this medium may cause the appearances of different colours. Franklin's Exper. and Obsery. 1769, p. 264.
The celebrated Eider has also mains tamed the same hypothesis, in his " Theo ria Lucis et Colorum." In the summary of his arguments against the common opinion, recited in Acad. Berl. 1752, p. 271, besides the objections above-men tioned, he doubts the possibility, that par tides of matter, moving with the amaz ing velocity of light, should penetrate transparent substances with so much ease. In whatever manner they are transmitted, those bodies must have pores, disposed in right lines, and in all possible directions, to serve as canals for the passage of the rays; but such a struc ture must take away all solid matter from those bodies, and all coherence among their parts, if they do contain any solid matter.
Among modern philosophers who have supported this doctrine, Dr. Young
has shown much ability in his capers. mental and theoretical researches, in his memoirs in the "Philosophical Trans actions," which have been republished in his " Lectures," and in " Nicholson's Journal." The expansion or extension of any por tion of light is inconceivable. Dr. Ilook shows, that it is as unlimited as the uni verse, which he proves from the im mense distance of many of the fixed stars, which only become visible to the eye by the best telescopes. " Nor," adds he, " are they only the great bodies of the sun or stars that are thus liable to dis perse their light through the vast expanse of the universe, but the smallest spark of a lucid body must do the same, even the smallest globule struck from a steel by a flint." The intensity of different lights, or of the same light in different circumstances, affords a curious subject of speculation.
M. Bouguer, Traite de Optique, found, that when one light is from sixty to eighty times less than another, its pre sence or absence will not be perceived by an ordinary eye ; that the moon's light,-when she is 19° 16' high above the horizon, is about one-third of her light, at 66° 11' high ; and'wh.en one limb just touched the horizon, her light was but the 2,000th part of her light at 66° 11' high ; and that hence light is diminished in the proportion of three to one, by tra versing 7.469 toises of dense air. He found also, that the centre of the sun's disc is considerably more luminous than the edges of it ; whereas both the prima ry and secondary planets are more lumi nous at their edges than near their cen tres : that, further, the light of the sun is about 300,000 times greater than that of the moon ; and therefore it is no wonder that philosophers have had so little suc cess in their attempts to collect the' light of the moon with burning glasses; for, should one of the largest of even increase the light 1,000 times, it will still leave the light of the moon in the focus of the glass, 300 times less than the intensity of the common light of the sun, Dr. Smith, in his optics, vol. i. p. thought he had proved that the light of the full moon would be only the 90,900th part of the full day-light, if no rays were lost at the moon. But Mr. Robins, in his Tracts, vol. ii. p. 225, shows that this is too great by one half. And Mr. Mitchell; by a more easy and accurate mode of computation, found that the density of the sun's light on the surface of the moon, is but the 45,000th part of the den. say at the sun ; and that, therefore, as the moon is nearly of the same apparent magnitude as the sun, if she reflected to us all the light received on her surface, it would be only the 45,000th part of our' day-light, or that which we receive from the sun. Admitting, therefore'vith M. I3ouguer, that the moon's light is only the 300,000th part of the day, or sun's light, Mr. Mitchell concludes that the moon reflects no more than between the 6th and 7th part of what she re ceives.