e pondering over the
familiar but puzzling phenomena of colored rings into which white
light is broken when reflected from thin films--Newton's rings, so
called--that an explanation occurred to him which at once put the entire
undulatory theory on a new footing. With that sagacity of insight which
we call genius, he saw of a sudden that the phenomena could be explained
by supposing that when rays of light fall on a thin glass, part of the
rays being reflected from the upper surface, other rays, reflected from
the lower surface, might be so retarded in their course through the
glass that the two sets would interfere with one another, the forward
pulsation of one ray corresponding to the backward pulsation of another,
thus quite neutralizing the effect. Some of the component pulsations of
the light being thus effaced by mutual interference, the remaining
rays would no longer give the optical effect of white light; hence the
puzzling colors.

Here is Young's exposition of the subject:

Of the Colors of Thin Plates

"When a beam of light falls upon two refracting surfaces, the partial
reflections coincide perfectly in direction; and in this case the
interval of retardation taken between the surfaces is to their radius as
twice the cosine of the angle of refraction to the radius.

"Let the medium between the surfaces be rarer than the surrounding
mediums; then the impulse reflected at the second surface, meeting a
subsequent undulation at the first, will render the particles of the
rarer medium capable of wholly stopping the motion of the denser and
destroying the reflection, while they themselves will be more strongly
propelled than if they had been at rest, and the transmitted light will
be increased. So that the colors by reflection will be destroyed, and
those by transmission rendered more vivid, when the double thickness or
intervals of retardation are any multiples of the whole breadth of
the undulations; and at intermediate thicknesses the effects will be
reversed according to the Newtonian observation.

"If the same proportions be found to hold good with respect to thin
plates of a denser medium, which is, indeed, not improbable, it will be
necessary to adopt the connected demonstrations of Prop. IV., but, at
any rate, if a thin plate be interposed between a rarer and a denser
medium, the colors by reflection and transmission may be expected to
change places."

OF THE COLORS OF THICK PLATES

"When a beam o