ESSAY I-Of LIGHT. WOULD it not be proper, in the first place, to lay down those laws of nature, by which the material world is governed, and which, when we come to consider, we have in the rank of second causes, no further to go? All mechanical accounts are at an end; we step into the glorious God immediately the very next thing we have to do, is to acknowledge Him, who is the first cause of all: and the Christian philosopher will on all invitations make the acknowledgments. The acute pen of Dr. Cheyne has thus delivered them. I. All bodies persevere in the same state of rest, or of moving forwards in a straight line, unless forced out of that state, by some violence outwardly impressed upon them. II. The changes made in the motions of bodies, are always proportional to the impressed force that moves them; and are produced in the same direction with that of the moving force. III. The same force with which one body strikes another, is returned upon the first by that other ; but these forces are impressed by contrary directions. IV. Every part of every body attracts or gravitates towards every part of every other body: but the force by which one part attracts another, in different distances from it, is reciprocally as the squares of those distances; and at the same distance, the force of the attraction or gravitation of one part towards divers others, is as the quantity of matter they contain. These are laws of the great God, who formed all things. God is ever to be seen in these everlasting ordinances. But now, in proceeding to magnify that work of God which men behold, it seems proper to begin with that by which we behold the rest. The light calls first for our contemplation. A most marvellous creature, whereof the great God is the father: by that alone we might discover the being of a God. The verus Christianismus of the pious John Arndt very well does insist upon that strain of piety; God and his love exhibited in the light. It was demanded, In what place is the light contained? By what way is the light divided? Aristotle's definition, "Light is in the inworking of a diaphanous body," is worth an attentive consideration. Light is undoubtedly produced, as Dr. Hook judges, by a motion, quick and vibrative. It is proved by Mr. Molyneux, that light is a body. Its refraction, in passing through a diaphanous body, shews that it finds a different resistance; resistance must proceed from a contact of two bodies. Moreover, it requires time to pass from one place to another, though it has indeed the quickest of all motions. Finally, it cannot by any means be increased or diminished. If you increase it, it is by robbing it of some other part of the medium which it would have occupied, or by bringing the light, that should naturally have been diffused through some other place, into that which is now more enlightened. Sir Isaac Newton judges, it is probable, that bodies and light act mutually on one another. Bodies upon light, in emitting it, and reflecting it, and refracting it, and inflecting it light upon bodies, by heating them, and putting their parts into a vibrating motion.. 1 All hypotheses of light are too dark, which try to explain the phenomena by new modifications of rays; they depend not on any such modifications, but on some congenite and unchangeable properties, essentially inherent in the rays. The rays of light are certainly little particles, actually emitted from the lucent body, and refracted by some attraction, by which light, and the bodies on which it falls, do mutually act upon one another. It is evident, that as rays pass by the edges of bodies, they are incurvated by the action of these bodies, as they pass by them. And it is now perceived, that bodies draw light, and this light puts bodies into heat and that the motion of light is therefore swifter in bodies, than in vacuo, because of this attraction; and slower after its being reflected, than in its incidence. Irradiated by the discoveries of the great Sir Isaac Newton, we now understand, that every ray of light is endowed with its own colour, and its different degree of refrangibility and reflexibility. One ray is violet, another indigo, a third blue, a fourth green, a fifth yellow, a sixth orange, and the last red. All these are original colours, and from the mixture of these, all the intermediate ones proceed; and white from an equable mixture of the whole: black, on the contrary, from the small quantity of any of them reflected, or all of them in a great measure suffocated. It is not bodies that are coloured, but the light that falls upon them; and their colours arise from the aptitude in them, to reflect rays of of one colour, and to transmit all those of another. It is now decided, no colour in the dark! Though light be certainly a body, it is almost impossible to conceive how small the corpuscles of it are. Dr. Cheyne illustrates it with an experiment, that it may be propagated from innumerable different luminous bodies, without any considerable opposition to one another. Their several streams of light will be together transmitted into a dark place, through the least orifice in the world. Suppose a plate of metal, having at the top the smallest hole that can be made, were erected perpendicularly upon an horizontal plane, and about it were set numberless luminous objects of about the same height with the plate, at an ordinary distance from it; the light proceeding from every one of these objects, will be propagated through this hole, without interfering. Mr. Romer, from his accurate observations of the eclipses on the Satellites of Jupiter, their immersions and emersions, thinks he has demonstrated, that light requires one second of time to move 9000 miles. He shews, that the rays of light require ten minutes of time to pass from the sun to us. And yet Mr. Hugens hath shewn, that a bullet from a cannon, without abating its first velocity, would be 25 years passing from us to the sun. So that the motion of light is above a million times swifter than that of a cannon ball; yea, we may carry the matter further. We suppose the distance of the sun from the earth to be 12000 diameters of the earth, or suppose 10000, the light then runs 1000 diameters in a minute; which is at least 130,000 miles in a second. Dr. Cheyne shews, that light is about 600,000 times more swift than sound. Amazing velocity! To checker the surprise at so swift a motion, I may propound one that shall be as very surpris ingly slow. Dee affirms, that he and Cardan together saw an instrument, in which there was one wheel constantly moving with the rest, and yet would not finish its revolution under the space of 7000 years. It is easy to conceive with Stevinus, an engine with 12 wheels, and the handle of such an engine to be turned about 4000 times in an hour, (which is as often as a man's pulse beats) yet in 10 years time the weight at the bottom would not move near so much as an hair's breadth and as Mersennus notes, it would not pass an inch in 1,000,000 years; although it be all this while in motion, and have not stood still one moment: for it is a mistake of Cardan, that very slow motions of necessity suppose intermediate rest. "Behold the light emitted from the sun; What more familiar, and what more unknown? The Jews have a good saying, The works of the external creation, possess the image of internal. |