LECTURE II.

  ORIGIN OF PHYSICAL THEORIES
  SCOPE OF THE IMAGINATION
  NEWTON AND THE EMISSION THEORY
  VERIFICATION OF PHYSICAL THEORIES
  THE LUMINIFEROUS ETHER
  WAVE THEORY OF LIGHT
  THOMAS YOUNG
  FRESNEL AND ARAGO
  CONCEPTION OF WAVE-MOTION
  INTERFERENCE OF WAVES
  CONSTITUTION OF SOUND-WAVES
  ANALOGIES OF SOUND AND LIGHT
  ILLUSTRATIONS OF WAVE-MOTION
  INTERFERENCE OF SOUND-WAVES
  OPTICAL ILLUSTRATIONS
  PITCH AND COLOUR
  LENGTHS OF THE WAVES OF LIGHT AND RATES OF VIBRATION OF
    THE ETHER-PARTICLES
  INTERFERENCE OF LIGHT
  PHENOMENA WHICH FIRST SUGGESTED THE UNDULATORY THEORY
  BOYLE AND HOOKE
  THE COLOURS OF THIN PLATES
  THE SOAP-BUBBLE
  NEWTON’S RINGS
  THEORY OF ‘FITS’
  ITS EXPLANATION OF THE RINGS
  OVER-THROW OF THE THEORY
  DIFFRACTION OF LIGHT
  COLOURS PRODUCED BY DIFFRACTION
  COLOURS OF MOTHER-OF-PEARL.

Sec. 1. Origin and Scope of Physical Theories.

We might vary and extend our experiments on Light indefinitely, and they certainly would prove us to possess a wonderful mastery over the phenomena.  But the vesture of the agent only would thus be revealed, not the agent itself.  The human mind, however, is so constituted that it can never rest satisfied with this outward view of natural things.  Brightness and freshness take possession of the mind when it is crossed by the light of principles, showing the facts of Nature to be organically connected.

Let us, then, inquire what this thing is that we have been generating, reflecting, refracting and analyzing.

In doing this, we shall learn that the life of the experimental philosopher is twofold.  He lives, in his vocation, a life of the senses, using his hands, eyes, and ears in his experiments:  but such a question as that now before us carries him beyond the margin of the senses.  He cannot consider, much less answer, the question, ’What is light?’ without transporting himself to a world which underlies the sensible one, and out of which all optical phenomena spring.  To realise this subsensible world the mind must possess a certain pictorial power.  It must be able to form definite images of the things which that world contains; and to say that, if such or such a state of things exist in the subsensible world, then the phenomena of the sensible one must, of necessity, grow out of this state of things.  Physical theories are thus formed, the truth of which is inferred from their power to explain the known and to predict the unknown.