{"id":1862,"date":"2012-01-09T15:00:30","date_gmt":"2012-01-09T03:00:30","guid":{"rendered":"https:\/\/blogs.otago.ac.nz\/emxphi\/?p=1862"},"modified":"2012-01-16T07:50:55","modified_gmt":"2012-01-15T19:50:55","slug":"hypotheses-and-newtons-rings","status":"publish","type":"post","link":"https:\/\/blogs.otago.ac.nz\/emxphi\/hypotheses-and-newtons-rings\/","title":{"rendered":"Hypotheses and Newton’s Rings"},"content":{"rendered":"

Kirsten Walsh writes…<\/strong><\/p>\n

In Ian Lawson\u2019s recent post<\/a>, he mentioned Hooke\u2019s work on colours in thin films.\u00a0 In this post, I\u2019ll look at how Newton used his hypotheses on light to build on Hooke\u2019s work in some interesting and important ways.<\/p>\n

In his optical work of the early 1670s, while Newton prefers theories to hypotheses<\/a>, he thinks that hypotheses are acceptable, even useful, for two purposes:<\/p>\n

    \n
  1. To \u2018illustrate\u2019 (i.e. provide an intuitively plausible explanation of) the theory; and<\/li>\n
  2. To \u2018suggest\u2019 experiments.<\/li>\n<\/ol>\n

    However, he insists that hypotheses should always be removed from the final version of the theory.\u00a0 Recall Newton\u2019s claim from his 1672 paper: \u201cI shall not mingle conjectures with certainties\u201d.<\/p>\n

    In December 1675, Newton wrote his paper, \u201cAn hypothesis explaining the Properties of Light\u201d. \u00a0Here, he published his hypotheses on the nature of light for the first time.\u00a0 To summarise them briefly:<\/p>\n

      \n
    1. There is an \u2018aethereal medium\u2019;<\/li>\n
    2. Aether vibrates, carrying sounds, smells and light;<\/li>\n
    3. Aether penetrates and passes through the pores of solid substances;<\/li>\n
    4. Light is neither the aether itself, nor the vibrations, but a substance that is propagated from \u2018lucid\u2019 bodies and travels through the aether;<\/li>\n
    5. Light warms the aether and the aether refracts the light; and<\/li>\n
    6. The rays (or bodies) of which light consists differ from one another physically.<\/li>\n<\/ol>\n

      In this paper, Newton claims that he is only discussing these hypotheses for the purposes of \u2018illuminating\u2019 his theory.\u00a0 Moreover, he does not assert that these hypotheses are true, and emphatically does not use them to support his theory.\u00a0 For example, when he discusses hypothesis (4), Newton is careful not to push too forcefully for any particular account of light.\u00a0 He says one might suppose light to be \u201can aggregate of various peripatetic qualities\u201d, or \u201cunimaginably small and swift\u201d corpuscles of various sizes, or \u201cany other corporeal emanation or impulse or motion of any other medium diffused through the body of the aether\u201d:<\/p>\n

        Onely whatever Light be, I would suppose, it consists of Successive rayes differing from one another in contingent circumstances, as bignes, forme or vigour…\u00a0 And further I would suppose it divers from the vibrations of the aether.<\/ol>\n

        In this paper, there is a notable emphasis on experiment.\u00a0 For example, when Newton discusses hypothesis (1), he gives an account of a new electrical experiment which seems to support his claim.\u00a0 And when he discusses hypothesis (3), he discusses the implications for Boyle\u2019s tadpole experiments.\u00a0 But the most important experiments in this paper are his investigations on the colours that appear between two glass surfaces.<\/p>\n

        Alan Shapiro<\/a> notes that Newton began these investigations while he was reading Hooke\u2019s Micrographia<\/a>.\u00a0 But his experiments and mathematical descriptions quickly developed into something well beyond the scope of Hooke\u2019s investigations.\u00a0 Hooke described the colours that appear when two thin sheets of glass are placed one on top of the other. \u00a0When he made the thin film of air between the two sheets thicker or thinner by pressing the two sheets together with greater or lesser force, the colours changed.\u00a0 He observed that different colours appeared at different thicknesses, but he was unable to quantify this observation as he was unable to measure accurately the thickness of the film at any given point.\u00a0 Newton had the idea of placing a convex lens on top of a flat sheet of glass.\u00a0 This enabled him to easily calculate the thickness of the film of air, and the colours appeared as a set of concentric coloured circles centred at the point of contact between the two surfaces.\u00a0 These concentric circles are now known as \u2018Newton\u2019s Rings\u2019.<\/p>\n

        \n
        \n
        \n
        \"\"

        Opticks, Book 2, Figure 3<\/p><\/div>\n<\/dt>\n

        <\/dd>\n<\/dl>\n<\/div>\n

         <\/p>\n

         <\/p>\n

         <\/p>\n

         <\/p>\n

         <\/p>\n

        Next Newton considered his hypotheses.\u00a0 According to hypothesis (2) the vibrations of the aether vary in size, according to hypothesis (3) aether passes through the pores of solid substances, and according to hypothesis (6) rays of different colours will cause aethereal vibrations of different sizes.\u00a0 If these hypotheses were correct, he argued, then light of a particular colour would be reflected either when the length of the vibration, or some multiple of the length of the vibration, matched the thickness of the film, and transmitted otherwise.\u00a0 So he predicted that:<\/p>\n

          if the Glasses in this posture be looked upon, there ought to appear at A <\/em>[the centre], the contact of the Glasses, a black spott, & about that many concentric circles of light & darknesse, the squares of whose semidiameters are to sense in arithmetical progression.<\/ol>\n

          Newton\u2019s \u201cHypothesis\u201d paper provides a good example of his method of hypotheses.\u00a0 He remains carefully detached from his own hypothesis,\u00a0using it only to \u2018illustrate\u2019 his theory and to suggest further experiments.\u00a0 Newton was also careful to keep his hypotheses well separate from his theory; the paper ends with a series of \u2018Observations\u2019 that contain no reference to his hypotheses at all!<\/p>\n","protected":false},"excerpt":{"rendered":"

          Kirsten Walsh writes… In Ian Lawson\u2019s recent post, he mentioned Hooke\u2019s work on colours in thin films.\u00a0 In this post, I\u2019ll look at how Newton used his hypotheses on light to build on Hooke\u2019s work in some interesting and important […]<\/p>\n","protected":false},"author":4582,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[113],"tags":[276,227,359,224,348],"class_list":["post-1862","post","type-post","status-publish","format-standard","hentry","category-ideas","tag-experiment","tag-history-of-science","tag-hypothesis","tag-newton","tag-optics"],"_links":{"self":[{"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/posts\/1862","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/users\/4582"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/comments?post=1862"}],"version-history":[{"count":0,"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/posts\/1862\/revisions"}],"wp:attachment":[{"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/media?parent=1862"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/categories?post=1862"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.otago.ac.nz\/emxphi\/wp-json\/wp\/v2\/tags?post=1862"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}