_Newton’s Discoveries respecting the Inflection or Diffraction of
Light—Previous Discoveries of Grimaldi and Dr. Hooke—Labours of
succeeding Philosophers—Law of Interference of Dr. Young—Fresnel’s
Discoveries—New Theory of Inflection on the Hypothesis of the
Materiality of Light._
Although the discoveries of Newton respecting the _Inflection of Light_
were first published in his _Optics_ in 1704, yet there is reason to
think that they were made at a much earlier period. Sir Isaac, indeed,
informs us, in his preface to that great work, that the third book,
which contains these discoveries, “was put together out of scattered
papers;” and he adds at the end of his observations, that “he designed
to repeat most of them with more care and exactness, and to make some
new ones for determining the manner how the rays of light are bent
in their passage by bodies, for making the fringes of colours with
the dark lines between them. But I was then interrupted, and cannot
now think of taking these things into consideration.” On the 18th
March, 1674, Dr. Hooke had read a valuable memoir on the phenomena of
diffraction; and, as Sir Isaac makes no allusion whatever to this work,
it is the more probable that his “scattered papers” had been written
previous to the communication of Dr. Hooke’s experiments.
The phenomena of the inflection of light were first discovered by
Francis Maria Grimaldi, a learned Jesuit, who has described them in a
posthumous work published in 1665, two years after his death.[29]
Having admitted a beam of the sun’s light through a small pin-hole in
a piece of lead or card into a dark chamber, he found that the light
diverged from this aperture in the form of a cone, and that the shadows
of all bodies placed in this light were not only larger than might have
been expected, but were surrounded with three coloured fringes, the
nearest being the widest, and the most remote the narrowest. In strong
light he discovered analogous fringes within the shadows of bodies,
which increased in number with the breadth of the body, and became
more distinct when the shadow was received obliquely and at a greater
distance. When two small apertures or pin-holes were placed so near
each other that the cones of light formed by each of them intersected
one another, Grimaldi observed, that a spot common to the circumference
of each, or, which is the same thing, illuminated by rays from each
cone, was darker than the same spot when illuminated by either of
the cones separately; and he announces this remarkable fact in the
following paradoxical proposition, “_that a body actually illuminated
may become more dark_ by adding a _light to that which it already
receives_.”
Without knowing what had been done by the Italian philosopher, our
countryman, Dr. Robert Hooke, had been diligently occupied with the
same subject. In 1672, he communicated his first observations to the
Royal Society, and he then spoke of his paper as “containing the
discovery of a new property of light not mentioned by any optical
writers before him.” In his paper of 1674, already mentioned, and which
is no doubt the one to which he alludes, he has not only described
the leading phenomena of the inflection, or the deflection of light,
as he calls it, but he has distinctly announced the _doctrine of
interference_, which has performed so great a part in the subsequent
history of optics.[30]
Such was the state of the subject when Newton directed to it his powers
of acute and accurate observation. His attention was turned only to the
enlargement of the shadow, and to the three fringes which surrounded
it; and he begins his observations by ascribing the discovery of these
facts to Grimaldi. After taking exact measures of the diameter of the
shadow of a human hair, and of the breadth of the fringes at different
distances behind it, he discovered the remarkable fact that these
diameters and breadths were not proportional to the distances from the
hair at which they were measured. In order to explain these phenomena,
Newton supposed that the rays which passed by the edge of the hair
are deflected or turned aside from it, as if by a repulsive force,
the nearest rays suffering the greatest, and those more remote a less
degree of deflection.
[Illustration: _Fig. 10._]
Thus, if X, fig. 10, represents a section of the hair, and AB, CD, EF,
GH, &c. rays passing at different distances from X, the ray AB will be
more deflected than CD, and will cross it at _m_, the ray CD will for
the same reason cross EF at _n_, and EF will cross GH at _o_. Hence the
curve or caustic formed by the intersections _m_, _n_, _o_, &c. will
be convex outward, its curvature diminishing as it recedes from the
vertex. As none of the passing light can possibly enter within this
curve, it will form the boundary of the shadow of X.
The explanation given by Sir Isaac of the coloured fringes is less
precise, and can be inferred only from the two following queries.
1. “Do not the rays which differ in refrangibility differ also in
flexibility, and are they not, by these different inflections separated
from one another, so as after separation to make the colours in the
three fringes above described? And after what manner are they inflected
to make those fringes?
2. “Are not the rays of light in passing by the edges and sides of
bodies _bent several times backwards and forwards_ with a motion like
that of an eel? And do not the three fringes of light above mentioned
arise from three such bendings?”
The idea thus indistinctly thrown out in the preceding queries has
been ingeniously interpreted by Mr. Herschel in the manner represented
in fig. 11, where SS are two rays passing by the edge of the body
MN. These rays are supposed to undergo several bendings, as at _a_,
_b_, _c_, and the particles of light are thrown off at one or other
of the points of contrary flexure, according to the state of their
fits or other circumstances. Those that are thrown outwards in the
direction _a_A, _b_B, _c_C, _d_D, will produce as many caustics by
their intersections as there are deflected rays; and each caustic, when
received on a screen at a distance, will depict on it the brightest
part or maximum of a fringe.
[Illustration: _Fig. 11._]
In this unsatisfactory state was the subject of the inflection of light
left by Sir Isaac. His inquiries were interrupted, and never again
renewed; and though he himself found that the phenomena were the same,
“whether the hair was encompassed with air or with any other pellucid
substance,” yet this important result does not seem to have shaken his
conviction, that the phenomena had their origin in the action of bodies
upon light.
During two sets of experiments which I made on the inflection of light,
the first in 1798, and the second in 1812 and 1813, I was desirous
of examining the influence of density and refractive power over the
fringes produced by inflection. I compared the fringes formed by
gold-leaf with those formed by masses of gold,—and those produced by
films which gave the colours of thin plates with those formed by masses
of the same substance. I examined the influence of platinum, diamond,
and cork in inflecting light, the effect of non-reflecting grooves
and spaces in polished metals, and of cylinders of glass immersed in
a mixture of oil of cassia and oil of olives of the same refractive
power; and, as the fringes had the same magnitude and character under
all these circumstances, I concluded that they were not produced by any
force inherent in the bodies themselves, but arose from a property of
the light itself, which always showed itself when light was stopped in
its progress.
Dr. Thomas Young, who had supported with great ingenuity and force
of argument the undulatory theory of light, as maintained by Hooke
and Huygens, was the first who gave a plausible explanation of the
inflection of light. By interposing a small screen at B, _fig. 10_, and
intercepting the rays that passed near the hair X, he was surprised to
find that all the _fringes within the shadow disappeared_. The same
effect took place when the screen intercepted the rays on the other
side; and hence he concluded, that the rays on each side of the hair
were necessary to the production of the inner fringes, and that the
fringes were produced by the _interference_ of the rays that passed
on one side of the hair with those that passed on the other side. In
order to account for the coloured fringes without the shadow, Dr.
Young conceived that the rays which pass near the edge of the hair
interfere with others, which he supposes may be reflected after falling
very obliquely upon its edge,—a supposition which, if correct, would
certainly produce fringes very similar to those actually observed.
In pursuing these researches so successfully begun by Dr. Young,
M. Fresnel had the good fortune to explain all the phenomena of
inflection by means of the undulatory doctrine combined with the
principle of interference. In place of transmitting the light through
a small aperture, he caused it to diverge from the focus of a deep
convex lens, and instead of receiving the shadow and its fringes upon
a smooth white surface, as was done by Newton, he viewed them directly
with his eye through a lens placed behind the shadow; and by means
of a microscope he was able to measure the dimensions of the fringes
with the greatest exactness. By this mode of observation he made the
remarkable discovery, that the inflection of the light _depended on
the distance of the inflecting body from the aperture or from the
focus of divergence_;[31] the fringes being observed to dilate as
the body approached that focus, and to contract as it receded from
it, their relative distances from each other, and from the margin of
the shadow continuing invariable. In attempting to account for the
formation of the exterior fringes, M. Fresnel found it necessary to
reject the supposition of Dr. Young, that they were owing to light
reflected from the edge of the body. He not only ascertained that the
real place of the fringe was the 17/100th of a millimetre different
from what it should be on that supposition, but he found that the
fringes preserved the same intensity of light, whether the inflecting
body had a round or a sharp edge, and even when the edge was such
as not to afford sufficient light for their production. From this
difficulty the undulatory theory speedily released him, and he was led
by its indications to consider the exterior fringes, as produced by an
infinite number of elementary waves of light emanating from a primitive
wave when partly interrupted by an opaque body.
The various phenomena of inflection, which had so long resisted every
effort to generalize them, having thus received so beautiful and
satisfactory an explanation from the undulatory doctrine, they must
of course be regarded as affording to that doctrine the most powerful
support, while the Newtonian hypothesis of the materiality of light is
proportionally thrown into the shade. It is impossible, indeed, even
for national partiality to consider the views of Newton as furnishing
any explanation of the facts discovered by Fresnel; and, as no attempt
has been made by the small though able phalanx of his disciples to
stay the decision with which, on this count at least, the doctrine
of emission has been threatened, we shall venture to suggest some
principles by which the refractory phenomena may perhaps be yet brought
within the pale of the Newtonian theory.
That the particles of light, like those of heat, are endowed with a
repulsive force which prevents them from accumulating when in a state
of condensation, or when they are detained by the absorptive action
of opaque bodies, will be readily admitted. By this power a beam of
light radiating from a luminous point has, in every azimuth, the same
degree of intensity at the same distance from its centre of divergence;
but if we intercept a portion of such a beam by an opaque body, the
repulsive force of the light which formerly occupied its shadow is
withdrawn, and consequently the rays which pass near the body will be
repelled into the shadow, and will form, by their interference with
those similarly repelled on the other side, the interior fringes, which
are parallel to the edges of the body. The rays which pass at a greater
distance will in like manner be bent towards the body, but with less
force, and, interfering with those rays which retain their primitive
direction, from the state of their fits or the position of their
poles, they will form the exterior fringes. When the inflecting body
is placed near the point of divergence, the greater proximity of the
rays will produce a greater repulsive force, and consequently a greater
inflection of the passing light; while the removal of the body from the
point of divergence will be accompanied with an increased distance of
the particles, an inferior repulsive force, and a feebler inflection.
As the phenomena of inflection, considered under this aspect, arise
from a property of the light itself, it follows that they will remain
invariable, whatever be the nature or density of the body, or the form
of the edge which acts upon the passing rays.