A History of Science - v.3 (Williams)

to furnish WITHOUT LIMITATION cannot possibly be a MATERIAL substance; and it appears to me to be extremely

difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated, in the manner the heat was excited and communicated

in these experiments, except in MOTION."[1]

THOMAS YOUNG AND THE WAVE THEORY OF LIGHT

But contemporary judgment, while it listened respectfully to Rumford, was little minded to accept his

verdict. The cherished beliefs of a generation are not to be put down with a single blow. Where many minds

have a similar drift, however, the first blow may precipitate a general conflict; and so it was here. Young

Humphry Davy had duplicated Rumford's experiments, and reached similar conclusions; and soon others fell into line. Then, in 1800, Dr. Thomas Young-- "Phenomenon Young" they called him at Cambridge, because he was reputed to know everything--took up

the cudgels for the vibratory theory of light, and it began to be clear that the two "imponderables," heat and light, must stand or fall together; but no one as yet made a claim against the fluidity of electricity.

Before we take up the details of the assault made by

Young upon the old doctrine of the materiality of light,

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Meantime languages had been but an incident in the education of the lad. He seems to have entered every available field of thought--mathematics, physics, botany, literature, music, painting, languages, philosophy, archaeology, and so on to tiresome lengths--and once

he had entered any field he seldom turned aside until he had reached the confines of the subject as then known and added something new from the recesses of his own genius. He was as versatile as Priestley, as profound

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as Newton himself. He had the range of a mere dilettante, but everywhere the full grasp of the master. He

took early for his motto the saying that what one man has done, another man may do. Granting that the

other man has the brain of a Thomas Young, it is a true motto.

Such, then, was the young Quaker who came to

London to follow out the humdrum life of a practitioner of medicine in the year 1801. But incidentally the young physician was prevailed upon to occupy the interims

of early practice by fulfilling the duties of the chair of Natural Philosophy at the Royal Institution, which

Count Rumford had founded, and of which Davy was then Professor of Chemistry--the institution whose

glories have been perpetuated by such names as Faraday and Tyndall, and which the Briton of to-day

speaks of as the "Pantheon of Science." Here it was that Thomas Young made those studies which have

insured him a niche in the temple of fame not far removed from that of Isaac Newton.

As early as 1793, when he was only twenty, Young

had begun to Communicate papers to the Royal Society of London, which were adjudged worthy to be printed

in full in the Philosophical Transactions; so it is not strange that he should have been asked to deliver the Bakerian lecture before that learned body the very first

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year after he came to London. The lecture was delivered November 12, 1801. Its subject was "The

Theory of Light and Colors," and its reading marks

an epoch in physical science; for here was brought forward for the first time convincing proof of that undulatory theory of light with which every student of

modern physics is familiar--the theory which holds

that light is not a corporeal entity, but a mere pulsation in the substance of an all-pervading ether, just as

sound is a pulsation in the air, or in liquids or solids.

Young had, indeed, advocated this theory at an

earlier date, but it was not until 1801 that he hit upon the idea which enabled him to bring it to anything approaching a demonstration. It was while 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.

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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

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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 of light passes through a refracting surface, especially if imperfectly polished, a portion of

it is irregularly scattered, and makes the surface visible in all directions, but most conspicuously in directions not far distant from that of the light itself; and if

a reflecting surface be placed parallel to the refracting surface, this scattered light, as well as the principal beam, will be reflected, and there will be also a new dissipation of light, at the return of the beam through the refracting surface. These two portions of scattered light will coincide in direction; and if the surfaces

be of such a form as to collect the similar effects, will exhibit rings of colors. The interval of retardation is here the difference between the paths of the principal beam and of the scattered light between the two surfaces; of course, wherever the inclination of the scattered

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By following up this clew with mathematical precision, measuring the exact thickness of the plate and

the space between the different rings of color, Young was able to show mathematically what must be the

length of pulsation for each of the different colors of the spectrum. He estimated that the undulations of red

light, at the extreme lower end of the visible spectrum, must number about thirty-seven thousand six hundred

and forty to the inch, and pass any given spot at a rate of four hundred and sixty-three millions of millions of undulations in a second, while the extreme violet numbers fifty-nine thousand seven hundred and fifty undulations to the inch, or seven hundred and thirty-five

millions of millions to the second.

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The Colors of Striated Surfaces

Young similarly examined the colors that are produced by scratches on a smooth surface, in particular

testing the light from "Mr. Coventry's exquisite micrometers," which consist of lines scratched on glass at

measured intervals. These microscopic tests brought the same results as the other experiments. The colors were produced at certain definite and measurable angles, and the theory of interference of undulations explained them perfectly, while, as Young affirmed with confidence, no other hypothesis hitherto advanced would explain them at all. Here are his

words:

"Let there be in a given plane two reflecting points very near each other, and let the plane be so situated that the reflected image of a luminous object seen in it

may appear to coincide with the points; then it is obvious that the length of the incident and reflected ray,

taken together, is equal with respect to both points, considering them as capable of reflecting in all directions. Let one of the points be now depressed below

the given plane; then the whole path of the light reflected from it will be lengthened by a line which is to

the depression of the point as twice the cosine of incidence to the radius.

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"If, therefore, equal undulations of given dimensions be reflected from two points, situated near enough to

appear to the eye but as one, whenever this line is equal to half the breadth of a whole undulation the reflection

from the depressed point will so interfere with the reflection from the fixed point that the progressive motion

of the one will coincide with the retrograde motion of the other, and they will both be destroyed; but when this line is equal to the whole breadth of an undulation, the effect will be doubled, and when to a breadth and a half, again destroyed; and thus for a

considerable number of alternations, and if the reflected undulations be of a different kind, they will be variously affected, according to their proportions to

the various length of the line which is the difference between the lengths of their two paths, and which may be denominated the interval of a retardation.

"In order that the effect may be the more perceptible, a number of pairs of points must be united into

two parallel lines; and if several such pairs of lines be placed near each other, they will facilitate the observation. If one of the lines be made to revolve

round the other as an axis, the depression below the given plane will be as the sine of the inclination; and while the eye and the luminous object remain fixed

the difference of the length of the paths will vary as this sine.

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"The best subjects for the experiment are Mr. Coventry's exquisite micrometers; such of them as consist

of parallel lines drawn on glass, at a distance of one- five-hundredth of an inch, are the most convenient.

Each of these lines appears under a microscope to consist of two or more finer lines, exactly parallel, and at a distance of somewhat more than a twentieth more than

the adjacent lines. I placed one of these so as to reflect the sun's light at an angle of forty-five degrees,

and fixed it in such a manner that while it revolved round one of the lines as an axis, I could measure its angular motion; I found that the longest red color

occurred at the inclination 10 1/4 degrees, 20 3/4 degrees, 32 degrees, and 45 degrees; of

which the sines are as the numbers 1, 2, 3, and 4. At

all other angles also, when the sun's light was reflected from the surface, the color vanished with the inclination, and was equal at equal inclinations on either side.

This experiment affords a very strong confirmation

of the theory. It is impossible to deduce any explanation of it from any hypothesis hitherto advanced;

and I believe it would be difficult to invent any other that would account for it. There is a striking analogy between this separation of colors and the production

of a musical note by successive echoes from equidistant iron palisades, which I have found to correspond pretty accurately with the known velocity of sound and the

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