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PHYSICAL OPTICS,

PART II.

THE CORPUSCULAR THEORY OF

LIGHT:

DISCUSSED MATHEMATICALLY.

BY

RICHARD POTTER, A.M. F.C.P.S.

LATE FELLOW OF QUEENS' COLLEGE, CAMBRIDGE; LICENTIATE OF THE ROYAL COLLEGE
OF PHYSICIANS, LONDON; HONORARY MEMBER OF THE LITERARY AND

PHILOSOPHICAL SOCIETY OF ST. ANDREW'S;

PROFESSOR OF NATURAL PHILOSOPHY AND ASTRONOMY

IN UNIVERSITY COLLEGE, LONDON.

CAMBRIDGE:
DEIGHTON, BELL, AND CO.

LONDON: BELL AND DALDY.

1859.

Cambridge: PRINTED BY C. J. CLAY, M. A.

AT THE UNIVERSITY PRESS.

PREFACE.

The proposal of a vibratory theory of light by Des Cartes was of little value until Huygens advanced to the undulatory theory, which he left nearly as it is at present received, but with the hypothesis of transversal vibrations added by Dr Young and M. Fresnel to account for the polarization of light. Newton, having adopted a mixed vibratory and corpuscular theory, was by his doctrine of the polarization of light enabled to give a reason for phenomena observed in the double refraction of calc spar which Huygens could not reconcile with an undulatory theory. Newton was, however, unfortunate in his hypothesis of 'fits of easy

reflexion and transmission,' formed to account for the occurrence of periodical colours; and his great authority during a century prevented the doctrine of interference, which had been advanced by Dr Hooke, from being accepted as the true explanation of the several cases investigated.

The Newtonian theory numbers, however, amongst its disciples the great names of Laplace and Malus, and the latter must ever rank as a chief leader in the advance of Physical Optics, by his mathematical investigations and his discovery of the polarization of light by reflexion at transparent surfaces.

The revival of the undulatory theory with the doctrine of interference, at the beginning of the present century, is due to Dr Young, and its reception by mathematicians to M. Fresnel's analytical researches, which promised, when fully developed, to comprehend the discoveries of himself, of M. Arago, M. Biot, Dr Brewster, M. Fraunhofer and others.

The expressing in mathematical formulæ the wonderful and beautiful phenomena shown in the interference of ordinary and polarized light, acted like an enchantment upon the mathematicians, and their glory and pride was to develop so surprising a theory. The inertia of the previous century came now into effect again in favour of the undulatory theory of light, and the investigations which professed to confirm or advance it were extolled, whilst those which militated against it were rejected or met by an assertion that at the utmost some subsidiary hypothesis might be needed.

In the experiment with the two mirrors slightly inclined, M. Arago had stated that the central interference-bar was always a bright one, in accordance with the undulatory theory; but it was found frequently to be seen a dark one, contrary to that theory, and means were sought to secure for exhibition a bright central bar. This was first accomplished by passing the light at the luminous point through a minute aperture in a thin opaque plate so that the light was in a state of interference by diffraction before falling upon the two

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mirrors, and the interference produced by them was secondary, but it was exhibited as primary interference. Another ingenious method to produce the same result is by forming the luminous point or line with lenses, so as to have interference near a caustic, before the light falls upon the two mirrors. In the case of the so-called spurious rainbows the measures of the bright bars did not accord well with the theory, whilst those of the dark bars did so; after full consultation, therefore, the former were suppressed, and the latter published.

With the reiteration of blunders and the management described above, together with bold assertions of the most complete agreement of the facts of experiment with conclusions from the undulatory theory, it was asserted to be as certainly true as the theory of gravitation.

With such advocacy it was not likely that the author of the present treatise would find companions in investigating critical points where the undulatory theory fails, and he has had the field nearly clear to himself for thirty years, until he had completed the experimental and mathematical investigations discussed in the Two Parts of the Work. The present Second Part was finished before the First Part was printed; but on account of the delay in getting it published he was enabled to revise, correct, and expand the manuscript to some small extent. The author is far from considering that he has done more than commence the mathe

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