urpose of repeating NEWTON'S experiments, and in
1810 he read the last of his three papers on the subject.

Sir ISAAC NEWTON had given some of his most vigorous efforts to the
study of the phenomena of interference of light, which are exemplified
in the colors of thin and of thick plates. The colors of thin plates are
most conveniently studied in the regular form which they present when
produced by a thin plate of air, limited on one side by a plane
polished surface, and on the other by a spherical surface of long
radius, such as the exterior surface of a convex lens, for example.
The colors are then arranged in concentric circles, and, though others
had so produced them before NEWTON, these rings have, ever since the
publication of his remarkable work, been known by his name.

To explain the phenomena, NEWTON was obliged to supplement his theory of
the corpuscular nature of light, by supposing that the inconceivably
minute particles constituting light are not always equally susceptible
of reflection, but that they have periodically recurring "fits of easy
reflection" and of "easy transmission." This conception, though by no
means unphilosophical, seemed to HERSCHEL too artificial and improbable
for ready acceptance, and his effort was to supply a more probable
explanation.

The developments of optical science have justified HERSCHEL in his
objections, but we cannot accord to him must any considerable part in
making clear the true nature of the phenomenon. Indeed, it must be
recognized that his position was distinctly less advanced than that of
NEWTON. That great philosopher announced the true law governing the
relation between the color and the thickness of the film. HERSCHEL did
not recognize such a relation. NEWTON showed exactly how the phenomenon
depended upon the obliquity at which it was viewed. HERSCHEL found no
place in his theory for this evident variation.

In the series of experiments described in the first paper on this
subject, HERSCHEL mistook the locus of a certain set of rings which he
was observing. This mistake, though so slight as hardly to be detected
without the guidance of the definite knowledge acquired in later times,
not only vitiated the conclusion from the experiments, but gave an
erroneous direction to the whole investigation. To him these experiments
proved that NEWTON'S conception of a periodic phenomenon was untenable.
Thus cut loose from all hypothesis, his fertility in ideas and inge