LIGHT

The revival and final establishment of the undulatory or wave theory of

light is one of the glories of the nineteenth century, and the credit

for it is due to Thomas Young, an Englishman, and Fresnel, a Frenchman.

Newton had conceived, espoused, and, owing to the great authority

of his name, almost fixed upon the learned world the corpuscular or

emission theory, which assumes that all luminous bodies emit streams of

minute corpuscles, which are reflected, refracted, and produce vision.

Many ordinary optical phenomena were explained by this hypothesis

only with great difficulty, and some were quite unexplainable. The

transmission of a disturbance or vibratory motion by means of waves, as

in the case of sound, was a well-recognized principle, and Young and

Fresnel applied it most successfully to the phenomena of light. Wave

motion, in a general way, is only possible in a sensibly continuous

medium, such as water, air, etc., and the theory that light was a

vibratory disturbance transmitted by means of waves necessitated the

assumption of the existence of such a medium throughout all space in

which light travelled. What is known as the ethereal medium, at first

a purely imaginary substance, but whose real existence is practically

established, satisfies this demand, and the hypothesis that light is

transmitted by waves in such a medium, originating in a vibratory

disturbance at the source, has been of inestimable value to physical

science.

The work of Thomas Young was done in the very first years of the

nineteenth century. He was for two years professor of Natural

Philosophy in the Royal Institution just founded by Count Rumford, and

he was the first to fill that chair. In 1801, in a paper presented to

the Royal Society, he argued in favor of the undulatory theory, showing

how the interference of waves would explain the color of thin plates.

His papers were not, for several years, received favorably, and they

were severely criticised by Lord Brougham. Augustus Fresnel followed

Young, but quite independently, about ten years later, and by him the

undulatory theory received elaborate experimental and mathematical

treatment.

In the mean time another Frenchman had made a capital discovery in

optics, which seemed at first to be quite incompatible with the wave

theory. This was the discovery of what is known as _polarization of

light_ by Malus, a French engineer, who hit upon it while investigating

double refraction of crystals, for a study of which the French

Institute had offered a prize in 1808. Malus found that when light fell

upon a surface of glass at a certain angle a portion of the reflected

light appeared to have acquired entirely new properties in regard to

further reflection, and the same was true of that part of the beam

which was transmitted through the glass. The light thus affected was

incapable of further reflection under certain conditions, and as the

beam seemed to behave differently according to how it was presented

to the reflecting surface, the term polarization was applied to the

phenomenon. It was found that the two rays into which a single beam

of light was split by a doubly refracting crystal (a phenomenon which

had long been known) were affected in this way, and that light was

polarized by refraction as well as by reflection. Malus was a believer

in the corpuscular theory of light, but it was shortly proved, first

by Thomas Young, that the phenomenon of polarization was not only

not opposed to the wave theory, but that that theory furnished a

rational explanation of it. This explanation, in brief, assumes that

ordinary light is a wave produced by a vibratory motion confined to

no particular plane, the direction of vibration being at right angles

to the direction of the wave, and in any, or, in rapid succession, in

_all_ azimuths. When light is polarized the vibratory motion in the

ether is restricted to one particular form, a line if plane polarized,

a circle or an ellipse if circularly or elliptically polarized. This

simple hypothesis has been found quite adequate, and through its

application to the various phenomena of polarization, together with

the application of Young’s theory of the interference of waves to

the production of color, the undulatory theory of light was firmly

established before the middle of the century. There were many noted

philosophers, however, who stood out long against it, notably Brewster,

the most famous English student of optics of the early part of the

century, who declared that his “chief objection to the undulatory

theory was that he could not think the Creator guilty of so clumsy a

contrivance as the filling of space with ether in order to produce

light.” In studying the nature of light it became very important to

know how fast a light wave travelled. A tolerably good measure of the

velocity of light had been made long before by means of the eclipses of

Jupiter’s moons and by observations upon the positions of the stars as

influenced by the motion of the earth in its orbit. It was found to be

approximately one hundred and eighty thousand miles per second, a speed

so great that it seemed impossible that it should ever be measured by

using only terrestrial distances.

This extremely difficult problem has been solved, however, in a most

satisfactory manner by nineteenth-century physicists. Everybody

knows that in a uniform motion velocity is equal to space or distance

divided by time. If, then, the time occupied in passing through a

given distance can be measured, the velocity is at once known. As the

velocity of light is very large, unless the distance is enormously

great, the time will be extremely small, and if moderate distances

are to be used the problem is to measure very small intervals of time

very accurately. Light will travel one mile in about the one hundred

and eighty-sixth thousandth part of a second, and if by using a mile

as the distance the velocity of light is to be determined within

one per cent., it is necessary to be able to detect differences of

time as small as about one twenty-millionth of a second. This has

been made possible by the use of two distinct methods. Foucault, on

the suggestion of Arago, used a rapidly revolving mirror, a method

introduced by Wheatstone, the English electrician, who used it in

finding the duration of an electric spark. The essential principle is

that a mirror may be made to revolve so rapidly that it will change its

position by a measurable angle, while light which has been reflected

from it passes to a somewhat distant fixed mirror and returns to the

moving reflector. In the other method a toothed wheel is revolved so

rapidly that a beam of light passing between two consecutive teeth to

a distant fixed mirror is cut off on its return to the wheel by the

tooth, which has moved forward while the light has made its journey.

This method was first used by Fizeau. In either method, if the speed

of rotation is known, the time is readily found. In point of time,

Fizeau was the first to attack the problem, which he did about 1849.

Foucault was perhaps a year later in getting results, but his method

is generally considered the best. Both methods have been used by other

experimenters, and very important improvements in Foucault’s method

were made in the United States by Michelson about 1878. Michelson’s

method increased enormously the precision of the measurements, and

it has been applied by him and by Newcomb, not only for the better

determination of the velocity of light in air, but for the solution

of many other related problems of first importance. Michelson’s final

determination of the absolute velocity of light (in the ether) is

everywhere accepted as authoritative.

Another discovery in optics entirely accomplished during the nineteenth

century and of the very first importance is generally known as

“Spectrum Analysis.” This discovery has not yet ceased to excite

admiration and even amazement, and especially among those who best

understand it. By its use hitherto unknown substances have become

known; to the physicist it is an instrument of research of the greatest

power, and perhaps more than anything else it promises to throw light

on the ultimate nature of matter; to the astronomer it has revealed

the composition, physical condition, and even the motions of the most

distant heavenly bodies, all of which the philosophy of a hundred years

ago would have pronounced absolutely impossible.

The beginning of spectrum analysis was in 1802, when an Englishman, Dr.

Wollaston, observed dark lines interrupting the solar spectrum when

produced by a good prism upon which the sunlight fell after passing

through a narrow slit. About ten years later, Fraunhofer, at Munich, a

skilful worker in glass and a keen observer, discovered in the spectrum

of light from a lamp two yellow bands, now known as the sodium, or

“D” lines. Combining the three essential elements of the modern

spectroscope, the slit, the prism, and the observing telescope, he saw

in the spectrum of sunlight “an almost countless number of dark lines.”

He was the first to use a grating for the production of the spectrum,

using at first fine wire gratings and afterwards ruling fine lines

upon glass, and with these he made the first accurate measures of the

length of light waves. He did not, however, comprehend the full import

of the problem which he thus brought to the attention of physicists.

About twenty years later Sir John Herschell studied the bright line

spectra of different substances and found that they might be used to

detect the presence of minute quantities of a substance whose spectrum

was known. Wheatstone studied the spectrum of the electric arc passing

between metals, and in 1874 Dr. J. W. Draper published a very important

paper on the spectra of solids with increasing temperature. Although

quite in the dark as to the real nature of the phenomena with which

they were dealing, these observers paved the way for the splendid work

of the two Germans, Kirchoff and Bunsen, who, about 1860, found the key

to this wonderful problem and made the science of spectrum analysis

substantially what it is to-day. Its fundamental principles may be

considered as few and comparatively simple.

Waves of light and radiant heat originate in ether disturbances

produced by molecular vibration, and have impressed upon them all of

the important qualities of that vibration. Molecules of different

substances differ in their modes of vibration, each producing a wave

peculiar to and characteristic of itself. A useful analogy may be found

in the fact that when one listens to the music of an orchestra without

seeing it it is easy to recognize the tones that come from each of

the several instruments, the characteristic vibrations of each being

impressed upon the waves in air which carry the sound to the ear. So

delicate and so sure is this impression of vibration peculiarities that

it is even possible to know the maker of a violin, for instance, by a

characteristic _timbre_ which must have its physical expression in the

sound wave. The ear, more perfect than the eye, analyzes the resultant

disturbance into its component parts so that each element may be

attributed to its proper source. Unaided, the eye cannot do this with

light, but the spectroscope separates the various modes of vibration

which make up the confused whole, so that varieties of molecular

activity are recognizable. The speed at which a source of sound is

approaching or receding from the ear can be ascertained by noting the

rise or fall in pitch due to the crowding together or stretching out

of the sound waves, and in the same way the motion of a luminous body

is known from the increase or decrease of the refrangibility of the

elements of its spectrum.

Indeed, had nineteenth-century science accomplished nothing else than

the discovery of spectrum analysis, it would have marked the beginning

of a new epoch. By this device man is put in communication with every

considerable body in the universe, including even the invisible. The

“goings on” of Sirius and Algol, of Orion and the Pleiads are reported

to him across enormous stretches of millions of millions of miles of

space, empty save of the ethereal medium itself, by this most wonderful

“wireless telegraphy.” And it is by the vibratory motion of the

invisibly small that all of this is revealed; the infinitely little has

enabled us to conquer the inconceivably big.

Many important contributions to the theory and practice of spectrum

analysis have been made since the time of Kirchoff and Bunsen, only

two or three of which can be referred to here. Instrumental methods by

which spectra are produced and examined have been greatly perfected,

and this is especially true of what is known as the “diffraction

grating” first used by Fraunhofer. A quarter of a century ago

Rutherford, of New York, constructed a ruling engine by means of which

gratings on glass and spectrum metal were ruled with a precision

greatly exceeding what had before been possible. A few years later

Rowland, of Baltimore, made a notable advance in the construction of

a screw far more perfect than any before made, producing gratings of

a fineness and regularity of spacing far ahead of any others, and

especially by the capital discovery of the concave grating, by means

of which the most beautiful results have been obtained. Very recently

Michelson, of Chicago, has invented the echelon spectroscope, which,

although greatly restricted in range, exceeds all others in power of

analysis of spectral lines. In his hands this instrument has been most

effective in the study of the influence of a strong magnetic field

upon the character of the spectrum from light produced therein, a most

interesting phenomenon first observed by Zeeman and one which promises

to reveal much concerning the relation of molecular activity to light

and to magnetic force.

The development of spectrum analysis was necessarily accompanied by a

recognition of the identity of radiant heat and light. The study of

radiant heat, which was carried on during the earlier years of the

century by Leslie, and later by Melloni and Tyndall, by what might be

called thermal methods, has been industriously pursued during the last

two decades by processes similar to those adopted for visual radiation.

The most notable contribution to this work is the invention of the

_bolometer_, by Langley, who, at Allegheny, and later at Washington,

has made exhaustive studies of solar radiation in invisible regions

of the spectrum, especially among the waves of greater length than

those of red light, where he has found absorption lines and bands

similar in character to those observed in the visible spectrum. He has

also studied the absorption of the earth’s atmosphere, the relation

of energy to visual effect, and many other interesting problems, the

solution of which was made possible by the use of the bolometer.

Mention must also be made of the invention by Michelson of an

interference comparator, by means of which linear measurements by

optical methods can be accomplished with a degree of accuracy hitherto

unheard of. With this instrument Michelson has determined the length of

the international prototype metre in terms of the wave length of the

light of a particular spectral line, thus furnishing for the first time

a satisfactory _natural_ unit of length.

By far the most important contribution to the theory of light made

during the last half of the century is that of Maxwell, who, in 1873,

announced the proposition that electro-magnetic phenomena and light

phenomena have their origin in the same medium, and that they are

identical in nature. This far-reaching conclusion has been generally

accepted and formed the basis of much of the most important work in

physical research in process of elaboration as the century closed. To

some of this reference will presently be made.