LITMIR.BIZ

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one another by refraction, and so cause the phenomena of prisms and other refracting substances ; and that it depends on the thickness of a thin transparent plate or bubble whether a vibration shall be reflected at its further superficies or transmitted; so that, according to the number of vibrations interceding the two superficies, they may be reflected or transmitted for many successive thicknesses. And since the vibrations which make blue and violet are supposed shorter than those which make red and yellow, they must be reflected at a less thickness of the plate, which is sufficient to explicate all the ordinary phenomena of those plates or bubbles, and [207] also of all natural bodies, whose parts are like so many fragments of such plates. These seem to be most plain, genuine, and necessary conditions of this hypothesis. And they agree so justly with my theory, that if the animadversor think lit to apply them, he need not on that account apprehend a divorce from it."^[3]

This passage is interesting as being the earliest presentation of a theory of color, now universally adopted. The same views were repeated as a suggestion, some forty-five years later, in the second edition of his treatise on "Optics."^[4]

In his "Letter to the Hon. Mr. Boyle," dated February 28, 1678–9, (about six years later,) Newton, after proposing as an explanation of the phenomena of cohesion, chemical affinity, &c., the "supposition" that an exceedingly elastic subtile aetherial substance is diffused through all places and bodies, but much rarer within and near gross bodies than beyond them, adds toward the conclusion of his letter: "I shall set down one conjecture more, which came into my mind now as I was writing this letter : it is about the cause of gravity. For this end I will suppose aether to consist of parts differing from one another in subtilty by indefinite degrees, … in such a manner that from the top of the air to the surface of the earth, and again from the surface of the earth to the center thereof, the aether is insensibly finer and finer. Imagine now any body suspended in the air or lying on the earth, and the £ether being by the hypothesis grosser in the pores which are in the upper parts of the body than in those which are in the lower parts, and that grosser aether being less apt to be lodged in those pores than the finer aether below, it will endeavor to get out, and give way to the finer a3ther below, which cannot be without the bodies descending to make room above for it to go into. From this supposed gradual subtilty of the parts of the aether, some things above might be further illustrated and made more intelligible. … For my own part, I have so little fancy to things of this nature, that had not your encouragement moved me to it, I should never I think have thus far set pen to ])aper about them."^[5] It will be seen from the above that Newton had not at this time (only three years before the crowning epoch of his life) extended his conception of "gravity" to the outlying universe.

Fourteen years later—a decade after his culminating work—this topic was again incidentally touched upon by Newton in four letters addressed to Doctor Bentley, "containing some arguments in proof of a Deity." In his second letter, dated January 17, 1692–3, he says in reply to one from Bentley : "You sometimes speak of gravity as essential and inherent to matter. Pray do not ascribe that notion to me, for the [208] cause of gravity is what I do not pretend to know, and therefore would take more time to consider of it."^[6]

In his third letter, dated February 25, 1692–3, he expresses himself somewhat less guardedly thus: "It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter, without mutual contact, as it must do if gravitation in the sense of Epicurus be essential and inherent in it. And this is one reason why I desired you would not ascribe 'innate gravity' to me. That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it. Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers."^[7]

      At the conclusion of the third book of his Principia, Newton remarks : " Hitherto I have not been able to discover the cause of those properties of gravity from phenomena, and I frame no hypothesis ; for whatever is not deduced from the phenomena is to be called an hypothesis. … To us it is enough that gravity does really exist, and act according to the laws which we have explained."

Still twenty-five years later than the date of these oft-quoted Bentley letters, Newton again recurred to the subject in an appendix to the second edition of his " Optics," published in 1717. After suggesting that the chromatic dispersion of luminous rays by refraction might be due to varying wave-lengths of an all-pervading " aetherial medium," (as previously referred to,) he asks : " Is not this medium much rarer within the dense bodies of the sun, stars, planets, and comets, than in the empty celestial spaces between them ? And in passing from them to greater distances, doth it not grow denser and denser perpetually, and thereby cause the gravity of those great bodies toward one another, and of their parts toward bodies ', every body endeavoring to go from the denser parts of the medium toward the rarer ? … And though this increase of density may at great distances be exceeding slow, yet if the elastic force of this medium be exceeding great, it may suffice to impel bodies from the denser parts of the medium toward the rarer, with all that power which we call gravity."^[8]

The intellectual spirit of the age in which "gravitation" was established was one of strong reaction from the previous metaphysical sway of "occult qualities;" and that the above crude suggestion (perhaps offered too much in deference to that spirit) by no means satisfied the judgment of Newton, is shown by his subsequent inclination to dispense [209] altogether with a medium which apparently must tend to retard the planetary movements, and which he though insufficient to account for the ordinary behavior of the luminous ray. He concludes that as "there is no evidence for its existence, therefore it ought to be rejected. And if it be rejected, the hypotheses that light consists in pression, or motion, propagated through such a medium, are rejected with it."^[9] This appears to have been the turning-point in the suspended balance of his judgment, determining his choice between the alternative conceptions of emission and of undulation.

Afterward, as if driven back from every assault to the only retreat. which in earlier years he had stigmatized as "so great an absurdity" that no competent thinker could "ever fall into it," he despairingly asks : " Have not the small particles of bodies certain powers, virtues, or forces, by which they act at a distance? … What I call 'attraction' may be performed by impulse, or by some other means unknown to me. I use that word here to signify only in general any force by which bodies tend toward one another, whatsoever be the cause."^[10] And beyond this point, no human research has since been able to penetrate.

      This last and presumably deliberate judgment of Newton is a quarter of a century later than the inconsiderate utterances of his third "Bentley letter," which have been so eagerly seized upon by every speculative writer intent on propounding new theories of the universe.

      The thoughtful philosopher Doctor Young, about a century later, commenting on Newton's suggestion of an aetherial medium—rarer toward and within dense bodies—with great ingenuity remarks: "The effects of gravitation might be produced by a medium thus constituted, if its particles were repelled by all material substances with a force decreasing like other repulsive forces, simply as the distances increase. Its density would then be everywhere such as to produce the appearance of an attraction varying like that of gravitation. Such an aetherial medium would therefore have the advantage of simplicity in the original law of its action, since the repulsive force which is known to belong to all matter would be sufficient, when thus modified, to account for the principal phenomena of attraction.

      "It may be questioned whether a medium capable of producing the effects of gravitation in this manner would also be equally susceptible of those modifications which we have supposed to be necessary for the transmission of light. In either case it must be supposed to pass through the apparent substance of all material bodies with the most perfect freedom, and there would therefore be no occasion to apprehend any difficulty from a retardation of the celestial motions, the ultimate impenetrable particles of matter being perhaps scattered as

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