Introductory Note
William Thomson, Baron Kelvin of Largs, was born in Belfast, Ireland, June 24,
1824. He was the son of the professor of mathematics at Glasgow University, and himself
entered that institution at the age of eleven. By the time he was twenty - one he
graduated from Cambridge as Second Wrangler, and, after studying in Paris, he returned to
Scotland to become, as professor of natural philosophy, the colleague of his father and
elder brother. The story of his life thenceforth is the record of amazingly brilliant and
fruitful scientific work, recognized by the award of almost all the honors appropriate to
such service, from learned societies, universities, and governments at home and abroad.
His part in laying the Atlantic Cable was the occasion of his receiving knighthood, and in
1892 he was raised to the peerage. He held his professorship at Glasgow for fifty - three
years, and later was chosen its Chancellor. He died on December 17, 1907, and was buried
in Westminster Abbey.
Lord Kelvin's activities were remarkable for both profundity and range. A large
number of his results are to be appreciated only by the highly skilled mathematician and
physicist; but his speculations on the ultimate constitution of matter; his statement of
the principle of the dissipation of energy, with its bearing upon the age of life on the
earth; his calculations as to the age of the earth itself, and much more, are of great
general interest. His fertility in practical invention was no less notable. He contrived a
large number of instruments; his services to navigation and ocean telegraphy being
especially valuable. Long before his death he was recognized as the most distinguished man
of science of his time and country, and he was also the most loved.
The lectures which follow are favorable examples of his power of exposition in
subjects in which he had no superior.
Part I.
[A Lecture Delivered At The Academy Of Music, Philadelphia, Under The Auspices Of
The Franklin Institute, September 29th, 1884.]
The subject upon which I am to speak to you this evening is happily for me not new in
Philadelphia. The beautiful lectures on light which were given several years ago by
President Morton, of the Stevens' Institute, and the succession of lecture, on the same
subject so admirably illustrated by Professor Tyndall, which many now present have heard,
have fully prepared you for anything I can tell you this evening in respect to the wave
theory of light.
It is indeed my humble part to bring before you only some mathematical and dynamical
details of this great theory. I cannot have the pleasure of illustrating them to you by
anything comparable with the splendid and instructive experiments which many of you have
already seen. It is satisfactory to me to know that so many of you, now present, are so
thoroughly prepared to understand anything I can say, that those who have seen the
experiments will not feel their absence at this time. At the same time I wish to make them
intelligible to those who have not had the advantages to be gained by a systematic course
of lectures. I must say, in the first place, without further preface, as time is short and
the subject is long, simply that sound and light are both due to vibrations propagated in
the manner of waves; and I shall endeavour in the first place to define the manner of
propagation and the mode of motion that constitute those two subjects of our senses, the
sense of sound and the sense of light.
Each is due to vibrations, but the vibrations of light differ widely from the
vibrations of sound. Something that I can tell you more easily than anything in the way of
dynamics or mathematics respecting the two classes of vibrations is, that there is a great
difference in the frequency of the vibrations of light when compared with the frequency of
the vibrations of sound. The term "frequency" applied to vibrations is a
convenient term, applied by Lord Rayleigh in his book on sound to a definite number of
full vibrations of a vibrating body per unit of time. Consider, then, in respect to sound,
the frequency of the vibrations of notes, which you all know in music represented by
letters, and by the syllables for singing, the do, re, mi, &c. The notes of the modern
scale correspond to different frequencies of vibrations. A certain note and the octave
above it, correspond to a certain number of vibrations per second, and double that number.
I may conveniently explain in the first place the note called 'C'; I mean the middle
'C'; I believe it is the C of the tenor voice, that most nearly approaches the tones used
in speaking. That note corresponds to two hundred and fifty - six full vibrations per
second - two hundred and fifty - six times to and fro per second of time.
Think of one vibration per second of time. The seconds pendulum of the clock performs
one vibration in two seconds, or a half vibration in one direction per second. Take a ten
- inch pendulum of a drawingroom clock, which vibrates twice as fast as the pendulum of an
ordinary eight - day clock, and it gives a vibration of one per second, a full period of
one per second to and fro. Now think of three vibrations per second. I can move my hand
three times per second easily, and by a violent effort I can move it to and fro five times
per second. With four times as great force, if I could apply it, I could move it twice
five times per second.
Let us think, then, of an exceedingly muscular arm that would cause it to vibrate ten
times per second, that is, ten times to the left and ten times to the right. Think of
twice ten times, that is, twenty times per second, which would require four times as much
force; three times ten, or thirty times a second, would require nine times as much force.
If a person were nine times as strong as the most muscular arm can be, he could vibrate
his hand to and fro thirty times per second, and without any other musical instrument
could make a musical note by the movement of his hand which would correspond to one of the
pedal notes of an organ.
If you want to know the length of a pedal pipe, you can calculate it in this way. There
are some numbers you must remember, and one of them is this. You, in this country, are
subjected to the British insularity in weights and measures; you use the foot and inch and
yard. I am obliged to use that system, but I apologize to you for doing so, because it is
so inconvenient, and I hope all Americans will do everything in their power to introduce
the French metrical system. I hope the evil action performed by an English minister whose
name I need not mention, because I do not wish to throw obloquy on any one, may be
remedied. He abrogated a useful rule, which for a short time was followed, and which I
hope will soon be again enjoined, that the French metrical system be taught in all our
national schools. I do not know how it is in America. The school system seems to be very
admirable, and I hope the teaching of the metrical system will not be let slip in the
American schools any more than the use of the globes. I say this seriously: I do not think
any one knows how seriously I speak of it. I look upon our English system as a wickedly
brain - destroying piece of bondage under which we suffer. The reason why we continue to
use it is the imaginary difficulty of making a change, and nothing else; but I do not
think in America that any such difficulty should stand in the way of adopting so
splendidly useful a reform.
I know the velocity of sound in feet per second. If I remember rightly, it is 1,089
feet per second in dry air at the freezing temperature, and 1,115 feet per second in air
of what we would call moderate temperature, 59 or 60 degrees - (I do not know whether that
temperature is ever attained in Philadelphia or not; I have had no experience of it, but
people tell me it is sometimes 59 or 60 degrees in Philadelphia, and I believe them) - in
round numbers let us call the speed 1,000 feet per second. Sometimes we call it a thousand
musical feet per second, it saves trouble in calculating the length of organ pipes; the
time of vibration in an organ pipe is the time it takes a vibration to run from one end to
the other and back. In an organ pipe 500 feet long the period would be one per second; in
an organ pipe ten feet long the period would be 50 per second; in an organ pipe twenty
feet long the period would be 25 per second at the same rate. Thus 25 per second, and 50
per second of frequencies correspond to the periods of organ pipes of 20 feet and 10 feet.
The period of vibration of an organ pipe, open at both ends, is approximately the time
it takes sound to travel from one end to the other and back. You remember that the
velocity in dry air in a pipe 10 feet long is a little more than 50 periods per second;
going up to 256 periods per second, the vibrations correspond to those of a pipe two feet
long. Let us take 512 periods per second; that corresponds to a pipe about a foot long. In
a flute, open at both ends, the holes are so arranged that the length of the sound wave is
about one foot, for one of the chief "open notes." Higher musical notes
correspond to greater and greater frequency of vibration, viz., 1,000, 2,000, 4,000,
vibrations per second; 4,000 vibrations per second correspond to a piccolo flute of
exceedingly small length; it would be but one and a half inches long. Think of a note from
a little dog - call, or other whistle, one and a half inches long, open at both ends, or
from a little key having a tube three quarters of an inch long, closed at one end; you
will then have 4,000 vibrations per second.
A wave - length of sound is the distance traversed in the period of vibration. I will
illustrate what the vibrations of sound are by this condensation travelling along our
picture on the screen. Alternate condensations and rarefactions of the air are made
continuously by a sounding body. When I pass my hand vigorously in one direction, the air
before it becomes dense, and the air on the other side becomes rarefied. When I move it in
the other direction these things become reversed; there is a spreading out of condensation
from the place where my hand moves in one direction and then in the reverse. Each
condensation is succeeded by a rarefaction. Rarefaction succeeds condensation at an
interval of one - half what we call "wave lengths." Condensation succeeds
condensation at the full interval of a wave length.
We have here these luminous particles on this scale,1 representing portions
of air close together, more dense; a little higher up, portions of air less dense. I now
slowly turn the handle of the apparatus in the lantern, and you see the luminous sectors
showing condensation travelling slowly upwards on the screen; now you have another
condensation making one wave length.
[Footnote 1: Alluding to a moving diagram of wave motion of sound produced by a working
slide for lantern projection.]
This picture or chart represents a wave - length of four feet. It represents a wave of
sound four feet long. The fourth part of a thousand is 250. What we see now of the scale
represents the lower note C of the tenor voice. The air from the mouth of a singer is
alternately condensed and rarefied just as you see here. But that process shoots forward
at the rate of about one thousand feet per second; the exact period of the motion being
256 vibrations per second for the actual case before you.
Follow one particle of the air forming part of a sound wave, as represented by these
moving spots of light on the screen; now it goes down, then another portion goes down
rapidly; now it stops going down; now it begins to go up; now it goes down and up again.
As the maximum of condensation is approached it is going up with diminishing maximum
velocity. The maximum of rarefaction has now reached it, and the particle stops going up
and begins to move down. When it is of mean density the particles are moving with maximum
velocity, one way or the other. You can easily follow these motions, and you will see that
each particle moves to and from and the thing that we call condensation travels along.
I shall show the distinction between these vibrations and the vibrations of light. Here
is the fixed appearance of the particles when displaced but not in motion. You can imagine
particles of something, the thing whose motion constitutes light. This thing we call the
luminiferous ether. That is the only substance we are confident of in dynamics. One thing
we are sure of, and that is the reality and substantiality of the luminiferous ether. This
instrument is merely a method of giving motion to a diagram designed for the purpose of
illustrating wave motion of light. I will show you the same thing in a fixed diagram, but
this arrangement shows the mode of motion.
Now follow the motion of each particle. This represents a particle of the luminiferous
ether, moving at the greatest speed when it is at the middle position.
You see the two modes of vibration,2 sound and light now moving together;
the travelling of the wave of condensation and rarefaction, and the travelling of the wave
of transverse displacement. Note the direction of propagation. Here it is from your left
to your right, as you look at it. Look at the motion when made faster. We have now the
direction reversed. The propagation of the wave is from right to left, again the
propagation of the wave is from left to right; each particle moves perpendicularly to the
line of propagation.
[Footnote 2: Showing two moving diagrams, simultaneously, on the screen, one depicting
a wave motion of light, the other a sound vibration.]
I have given you an illustration of the vibration of sound waves, but I must tell you
that the movement illustrating the condensation and rarefaction represented in that moving
diagram are necessarily very much exaggerated, to let the motion be perceptible, whereas
the greatest condensation in actual sound motion is not more than one or two percent, or a
small fraction of a percent. Except that the amount of condensation was exaggerated in the
diagram for sound, you have in the chart a correct representation of what actually takes
place in sounding the low note C.
On the other hand, in the moving diagram representing light waves what had we? We had a
great exaggeration of the inclination of the line of particles. You must first imagine a
line of particles in a straight line, and then you must imagine them disturbed into a wave
- curve, the shape of the curve corresponding to the disturbance. Having seen what the
propagation of the wave is, look at this diagram and then look at that one. This, in
light, corresponds to the different sounds I spoke of at first. The wave - length of light
is the distance from crest to crest of the wave, or from hollow to hollow. I speak of
crests and hollows, because we have a diagram of ups and downs as the diagram is placed.
Here, then, you have a wave - length.3 In this lower diagram (Fig. 119) you
have a wave - length of violet light. It is but one - half the length of the upper wave of
red light; the period of vibration is but half as long. Now there, on an enormous scale,
exaggerated not only as to slope, but immensely magnified as to wave - length, we have an
illustration of the waves of violet light. The drawing marked "red" (Fig. 118)
corresponds to red light, and this lower diagram corresponds to violet light. The upper
curve really corresponds to something a little below the red ray of light in the spectrum,
and the lower curve to something beyond the violet light. The variation in wave length
between the most extreme rays is in the proportion of four and a half of red to eight of
the violet, instead of four and eight; the red waves are nearly as one to two of the
violet.
[Footnote 3: Exhibiting a large drawing, or chart, representing a red and a violet wave
of light (reproduced in Figs. 118 and 119).]
To make a comparison between the number of vibrations for each wave of sound and the
number of vibrations constituting light waves, I may say that 30 vibrations per second is
about the smallest number which will produce a musical sound; 50 per second gives one of
the grave pedal notes of an organ, 100 or 200 per second give the low notes of the bass
voice, higher notes with 250 per second, 300 per second, 1,000, 4,000 up to 8,000 per
second give about the shrillest notes audible to the human ear.
Instead of the numbers, which we have, say in the most commonly used part of the
musical scale, i.e., from 200 or 300 to 600 or 700 per second, we have millions of
millions of vibrations per second in light waves; that is to say, 400 per second, instead
of 400 million million per second, which is the number of vibrations performed when we
have red light produced.
An exhibition of red light travelling through space from the remotest star is due to
propagation by waves or vibrations, in which each individual particle of the transmitting
medium vibrates to and fro 400 million million times in a second.
Some people say they cannot understand a million million. Those people cannot
understand that twice two makes four. That is the way I put it to people who talk to me
about the incomprehensibility of such large numbers. I say finitude is incomprehensible,
the infinite in the universe is comprehensible. Now apply a little logic to this. Is the
negation of infinitude incomprehensible? What would you think of a universe in which you
could travel one, ten, or a thousand miles, or even to California, and then find it come
to an end? Can you suppose an end of matter or an end of space? The idea is
incomprehensible. Even if you were to go millions and millions of miles the idea of coming
to an end is incomprehensible. You can understand one thousand per second as easily as you
can understand one per second. You can go from one to ten, and ten times ten and then to a
thousand without taxing your understanding and then you can go on to a thousand million
and a million million. You can all understand it.
Now 400 million million vibrations per second is the kind of thing that exists as a
factor in the illumination by red light. Violet light, after what we have seen and have
had illustrated by that curve (Fig. 119), I need not tell you corresponds to vibrations of
about 800 million million per second. There are recognisable qualities of light caused by
vibrations of much greater frequency and much less frequency than this. You may imagine
vibrations having about twice the frequency of violet light, and others having about one
fifteenth the frequency of red light, and still you do not pass the limit of the range of
continuous phenomena, only a part of which constitutes visible light.
When we go below visible red light what have we? We have something we do not see with
the eye, something that the ordinary photographer does not bring out on his
photographically sensitive plates. It is the light, but we do not see it. It is something
so closely continuous with visible light, that we may define it by the name of invisible
light. It is commonly called radiant heat; invisible radiant heat. Perhaps, in this thorny
path of logic, with hard words flying in our faces, the least troublesome way of speaking
of it is to call it radiant heat. The heat effect you experience when you go near a bright
hot coal fire, or a hot steam boiler; or when you go near; but not over, a set of hot
water pipes used for heating a house; the thing we perceive in our faces and hands when we
go near a boiling pot and hold the hand on a level with it, is radiant heat; the heat of
the hands and face caused by a hot fire, or by a hot kettle when held under the kettle, is
also radiant heat.
You might readily make the experiment with an earthen teapot; it radiates heat better
than polished silver. Hold your hands below the teapot and you perceive a sense of heat;
above it you get more heat; either way you perceive heat. If held over the teapot you
readily understand that there is a little current of hot air rising; if you put your hand
under the teapot you find cold air rising, and the upper side of your hand is heated by
radiation while the lower side is fanned and is actually cooled by virtue of the heated
kettle above it.
That perception by the sense of heat, is the perception of something actually
continuous with light. We have knowledge of rays of radiant heat perceptible down to (in
round numbers) about four times the wave - length, or one - fourth the period, of visible
or red light. Let us take red light at 400 million million vibrations per second, then the
lowest radiant heat, as yet investigated, is about 100 million million per second of
frequency of vibration.
I had hoped to be able to give you a lower figure. Professor Langley has made splendid
experiments on the top of Mount Whitney, at the height of 15,000 feet above the sea -
level, with his "Bolometer," and has made actual measurements of the wave -
length of radiant heat down to exceedingly low figures. I will read you one of the
figures; I have not got it by heart yet, because I am expecting more from him.4 I learned a year and a half ago that the lowest radiant heat observed by the diffraction
method of Professor Langley corresponds to 28 one hundred thousandths of a centimetre of
wave length, 28 as compared with red light, which is 7.3; or nearly four - fold. Thus wave
- lengths of four times the amplitude, or one - fourth the frequency per second of red
light, have been experimented on by Professor Langley and recognized as radiant heat.
[Footnote 4: Since my lecture I have heard from Professor Langley that he has measured
the refrangibility by a rock salt prism, and inferred the wave length of heat rays from a
"Leslie cube" (a metal vessel filled with hot water and radiating heat from a
blackened side). The greatest wave - length he has thus found is one - thousandth of a
centimetre, which is seventeen times that of sodium light - the corresponding period being
about thirty million million per second. November, 1884. - W. T.]
Everybody knows the "photographer's light," and has heard of invisible light
producing visible effects upon the chemically prepared plate in the camera. Speaking in
round numbers, I may say that, in going up to about twice the frequency I have mentioned
for violet light, you have gone to the extreme end of the range of known light of the
highest rates of vibration; I mean to say that you have reached the greatest frequency
that has yet been observed. Photographic, or actinic light, as far as our knowledge
extends at present, takes us to a little less than one - half the wave - length of violet
light.
You will thus see that while our acquaintance with wave motion below the red extends
down to one quarter of the slowest rate which affects the eye, our knowledge of vibrations
at the other end of the scale only comprehends those having twice the frequency of violet
light. In round numbers we have 4 octaves of light, corresponding to 4 octaves of sound in
music. In music the octave has a range to a note of double frequency. In light we have one
octave of visible light, one octave above the visible range and two octaves below the
visible range. We have 100 per second, 200 per second, 400 per second (million million
understood) for invisible radiant heat; 800 per second for visible light, and 1,600 per
second for invisible or actinic light.
One thing common to the whole is the heat effect. It is extremely small in moonlight,
so small that until recently nobody knew there was any heat in the moon's rays. Herschel
thought it was perceptible in our atmosphere by noticing that it dissolved away very light
clouds, an effect which seemed to show in full moonlight more than when we have less than
full moon. Herschel, however, pointed this out as doubtful; but now, instead of its being
a doubtful question, we have Professor Langley giving as a fact that the light from the
moon drives the indicator of his sensitive instrument clear across the scale, showing a
comparatively prodigious heating effect!
I must tell you that if any of you want to experiment with the heat of the moonlight,
you must measure the heat by means of apparatus which comes within the influence of the
moon's rays only. This is a very necessary precaution; if, for instance, you should take
your Bolometer or other heat detector from a comparatively warm room into the night air,
you would obtain an indication of a fall in temperature owing to this change. You must be
sure that your apparatus is in thermal equilibrium with the surrounding air, then take
your burning - glass, and first point it to the moon and then to space ion the sky beside
the moon; you thus get a differential measurement in which you compare the radiation of
the moon with the radiation of the sky. You will then see that the moon has a distinctly
heating effect.
To continue our study of visible light, that is undulations extending from red to
violet in the spectrum (which I am going to show you presently), I would first point out
on this chart. (Fig. 120) that in the section from letter A to letter D we have visual
effect and heating effect only; but no ordinary chemical or photographic effect.
Photographers can leave their usual sensitive chemically prepared plates exposed to yellow
light and red light without experiencing any sensible effect; but when you get toward the
blue end of the spectrum the photographic effect begins to tell, and more and more
strongly as you get towards the violet end. When you get beyond the violet there is the
invisible light known chiefly by its chemical action. From yellow to violet we have visual
effect, heating effect, and chemical effect, all three; above the violet only chemical and
heating effects, and so little of the heating effect that it is scarcely perceptible.
The prismatic spectrum is Newton's discovery of the composition of white light. White
light consists of every variety of colour from red to violet. Here, now, we have Newton's
prismatic spectrum, produced by a prism. I will illustrate a little in regard to the
nature of colour by putting something before the light which is like coloured glass; it is
coloured gelatin. I will put in a plate of red gelatin which is carefully prepared of
chemical materials and see what that will do. Of all the light passing to it from violet
to red it only lets through the red and orange, giving a mixed reddish colour. Here is a
plate of green gelatin: the green absorbs all the red, giving only green. Here is a plate
absorbing something from each portion of the spectrum, taking away a great deal of the
violet and giving a yellow or orange appearance to the light. Here is another absorbing
the green and all the violet, leaving red, orange, and a very little faint green.
When the spectrum is very carefully produced, four more carefully than Newton know how
to show it, we have a homogeneous spectrum. It must be noticed that Newton did not
understand what we call a homogeneous spectrum; he did not produce it, and does not point
out in his writings the conditions for producing it. With an exceedingly fine line of
light we can bring it out as in sunlight, like this upper picture - red, orange, yellow,
green, blue, indigo, and violet, according to Newton's nomenclature. Newton never used a
narrow beam of light, and so could not have had a homogeneous spectrum.
This is a diagram painted on glass and showing the colours as we know them. It would
take two or three hours if I were to explain the subject of spectrum analysis to - night.
We must tear ourselves away from it. I will just read out to you the wave - lengths
corresponding to the different positions of the sun's spectrum of certain dark lines
commonly called "Fraunhofer's lines." I will take as a unit the one hundred
thousandth of a centimetre. A centimetre is .4 of an inch; it is a rather small half an
inch. I take the thousandth of a centimetre and the hundredth of that as a unit. At the
red end of the spectrum the light in the neighbourhood of that black line A (Fig. 120) has
for its wave - length 7.6; B has 6.87; D has 5.89; the "frequency' for A is 3.9 times
100 million million, the frequency of D light is 5.1 times 100 million million per second.
Part II.
Now what force is concerned in those vibrations as compared with sound at the rate of
400 vibrations per second? Suppose for a moment the same matter was to move to and fro
through the same range but 400 million million times per second. The force required is as
the square of the number expressing the frequency. Double frequency would require
quadruple force for the vibration of the same body. Suppose I vibrate my hand again, as I
did before. If I move it once per second a moderate force is required; for it to vibrate
ten times per second 100 times as much force is required; for 400 vibrations per second
160,000 times as much force. If I move my hand once per second through a space of a
quarter of an inch a very small force is required; it would require; very considerable
force to move it ten times a second, even through so small a range; but think of the force
required to move a tuning fork 400 times a second, and compare that with the force
required for a motion of 400 million million times a second. If the mass moved is the
same, and the range of motion is the same, then the force would be one million million
million million times as great as the force required to move the prongs of the tuning fork
- it is as easy to understand that number as any number like 2, 3, or 4. Consider now what
that number means and what we are to infer from it. What force is there in the space
between my eye and that light? What forces are there in the space between our eyes and the
sun, and our eyes and the remotest visible star? There is matter and there is motion, but
what magnitude of force may there be?
I move through this "luminiferous ether" as if it were nothing. But were
there vibrations with such frequency in a medium of steel or brass, they would be measured
by millions and millions and millions of tons' action on a square inch of matter. There
are no such forces in our air. Comets make a disturbance in the air, and perhaps the
luminiferous ether is split up by the motion of a comet through it. So when we explain the
nature of electricity, we explain it by a motion of the luminiferous ether. We cannot say
that it is electricity. What can this luminiferous ether be? It is something that the
planets move through with the greatest ease. It permeates our air; it is nearly in the
same condition, so far as our means of judging are concerned, in our air and in the inter
- planetary space. The air disturbs it but little; you may reduce air by air - pumps to
the hundred thousandth of its density, and you make little effect in the transmission of
light through it. The luminiferous ether is an elastic solid, for which the nearest
analogy I can give you is this jelly which you see,5 and the nearest analogy to
the waves of light is the motion, which you can imagine, of this elastic jelly, with a
ball of wood floating in the middle of it. Look there, when with my hand I vibrate the
little red ball up and down, or when I turn it quickly round the vertical diameter,
alternately in opposite directions; - that is the nearest representation I can give you of
the vibrations of luminiferous ether.
[Footnote 5: Exhibiting a large bowl of clear jelly with a small red wooden ball
embedded in the surface near the centre.]
Another illustration is Scottish shoemakers' wax or Burgundy pitch, but I know Scottish
shoemakers' wax better. It is heavier than water, and absolutely answers my purpose. I
take a large slab of the wax, place it in a glass jar filled with water, place a number of
corks on the lower side and bullets on the upper side. It is brittle like the Trinidad
pitch or Burgundy pitch which I have in my hand - you can see how hard it is - but when
left to itself it flows like a fluid. The shoemakers' wax breaks with a brittle fracture,
but it is viscous and gradually yields.
What we know of the luminiferous ether is that it has the rigidity of a solid and
gradually yields. Whether or not it is brittle and cracks we cannot yet tell, but I
believe the discoveries in electricity and the motions of comets and the marvellous spurts
of light from them, tend to show cracks in the luminiferous ether - show a correspondence
between the electric flash and the aurora borealis and cracks in the luminiferous ether.
Do not take this as an assertion, it is hardly more than a vague scientific dream: but you
may regard the existence of the luminiferous ether as a reality of science; that is, we
have an all - pervading medium, an elastic solid, with a great degree of rigidity - an
rigidity so prodigious in proportion to its density that the vibrations of light in it
have the frequencies I have mentioned, with the wave - lengths I have mentioned. The
fundamental question as to whether or not luminiferous ether has gravity has not been
answered. We have no knowledge that the luminiferous ether is attracted by gravity; it is
sometimes called imponderable because some people vainly imagine that it has no weight; I
call it matter with the same kind of rigidity that this elastic jelly has.
Here are two tourmalines; if you look through them toward the light you see the white
light all round, i.e. they are transparent. If I turn round one of these tourmalines the
light is extinguished, it is absolutely black, as though the tourmalines were opaque. This
is an illustration of what is called polarisation of light. I cannot speak to you about
qualities of light without speaking of the polarisation of light. I want to show you a
most beautiful effect of polarising light, before illustrating a little further by means
of this large mechanical illustration which you have in the bowl of jelly. What you saw
first were two plates of the crystal tourmaline (which came from Brazil, I believe) having
the property of letting light pass when both plates are placed in one particular direction
as regards their axes of crystallisation, and extinguishing it when it passes through them
with one of the plates held in another direction. Now I put in the lantern an instrument
called a "Nicol prism," which also gives rays of polarised light. A Nicol prism
is a piece of Iceland spar, cut in two and turned one part relatively to the other in a
very ingenious way, and put together again and cemented into one by Canada balsam. The
Nicol prism takes advantage of the property which the spar has of double refraction, and
produces the phenomenon which I now show you. I turn one prism round in a certain
direction and you get light - a maximum of light. I turn it through a right angle and you
get blackness. I turn it one quarter round again, and get maximum light; one quarter more,
maximum blackness; one quarter more, and bright light. We rarely have a grand specimen of
a Nicol prism as this.
There is another way of producing polarised light. I stand before that light and look
at its reflection in a plate of glass on the table through one of the Nicol prisms, which
I turn round, so. Now if I incline that plate of glass at a particular angle - rather more
than fifty - five degrees - I find a particular position in which, if I look at it and
then turn the prism round in the hand, the effect is absolutely to extinguish the light in
one position of the prism and to give it maximum brightness in another position. I use the
term "absolute" somewhat rashly. It is only a reduction to a very small quantity
of light, not an absolute annulment as we have in the case of the two Nicol prisms used
conjointly. As to the mechanics of the thing, those of you who have never heard of this
before would not know what I was talking about; it could only be explained to you by a
course of lectures in physical optics. The thing is this, vibrations of light must be in a
definite direction relatively to the line in which the light travels.
Look at this diagram, the light goes from left to right; we have vibrations
perpendicular to the line of transmission. There is a line up and down which is the line
of vibration. Imagine here a source of light, violet light, and here in front of it is the
line of propagation. Sound - vibrations are to and fro in, this is transverse to, the line
of propagation. Here is another, perpendicular to the diagram, still following the law of
transverse vibration; here is another, circular vibration. Imagine a long rope, you whirl
one end of it and you see a screw - like motion running along, and you can get this
circular motion in one direction or in the opposite.
Plane - polarised light is light with the vibrations all in a single plane,
perpendicular to the plane through the ray which is technically called the "plane of
polarisation." Circularly polarised light consists of undulations of luminiferous
ether having a circular motion. Elliptically polarised light is something between the two,
not in a straight line, and not in a circular line; the course of vibration is an ellipse.
Polarised light is light that performs its motions continually in one mode or direction.
If in a straight line it is plane - polarised; if in a circular direction it is circularly
polarised light; when elliptical it is elliptically polarised light.
With Iceland spar, one unpolarised ray of light divides on entering it into two rays of
polarised light, by reason of its power of double refraction, and the vibrations are
perpendicular to one another in the two emerging rays. Light is always polarised when it
is reflected from a plate of unsilvered glass, or from water, at a certain definite angle
of fifty - six degrees for glass, fifty - two degrees for water, the angle being reckoned
in each case from a perpendicular to the surface. The angle for water is the angle whose
tangent is 1.4. I wish you to look at the polarisation with your own eyes. Light from
glass at fifty - six degrees and from water at fifty - two degrees goes away vibrating
perpendicularly to the plane of incidence and plane of reflection.
We can distinguish it without the aid of an instrument. There is a phenomenon well
known in physical optics as "Haidinger's Brushes." The discoverer is well known
in Philadephia as a mineralogist, and the phenomenon I speak of goes by his name. Look at
the sky in a direction of ninety degrees from the sun, and you will see a yellow and blue
cross, with the yellow toward the sun, and from the sun, spreading out like two foxes'
tails with blue between, and then two red brushes in the space at right angles to the
blue. If you do not see it, it is because your eyes are not sensitive enough, but a little
training will give them the needed sensitiveness. If you cannot see it in this way try
another method. Look into a pail of water with a black bottom; or take a clear glass dish
of water, rest it on a black cloth, and look down at the surface of the water on a day
with a white cloudy sky (if there is such a thing ever to be seen in Philadelphia). You
will see the white sky reflected in the basin of water at an angle of about fifty degrees.
Look at it with the head tipped on one side and then again with the head tipped to the
other side, keeping your eyes on the water, and you will see Haidinger's brushes. Do not
do it fast or you will make yourself giddy. The explanation of this is the refreshing of
the sensibility of the retina. The Haidinger's brush is always there, but you do not see
it because your eye is not sensitive enough. After once seeing it you always see it; it
does not thrust itself inconveniently before you when you do not want to see it. You can
also readily see it in a piece of glass with a dark cloth below it, or in a basin of
water.
I am going to conclude by telling you how we know the wave - lengths of light, and how
we know the frequency of the vibrations, and we shall actually make a measurement of the
wave - length of yellow light. I am now going to show you the diffraction spectrum.
You see on the screen,6 on each side of a central white bar of light, a set
of bars of light, of variegated colours, the first one on each side showing blue or indigo
colour, about four inches from the central white bar, and red about four inches farther,
with vivid green between the blue and the red. That effect is produced by a grating with
400 lines to the centimetre, engraved on glass, which I now hold in my hand. The next
grating that we shall try has 3,000 lines on a Paris inch. You see the central space and
on each side a large number of spectrums, blue at one end and red at the other. The fact
that, in the first spectrum, red is about twice as far from the centre as the blue, proves
that a wave - length of red light is double that of blue light.
[Footnote 6: Showing the chromatic bands thrown upon the screen from a diffraction
grating.]
I will now show you the operation of measuring the length of a wave of sodium light,
that is a light like that marked D on the spectrum (Fig. 120), a light produced by a
spirit - lamp with salt in it. The sodium vapour is heated up to several thousand degrees,
when it becomes self - luminous and gives such a light as we get by throwing salt upon a
spirit - lamp in the game of snap dragon.
I hold in my hand a beautiful grating of glass silvered by Liebig's process with
metallic silver, a grating with 6,480 lines to the inch, belonging to my friend Professor
Barker, which he has kindly brought here for us this evening. You will see the brilliancy
of colour as I turn the light reflected from the grating toward you and pass the beam
round the room. You have now seen directly with your own eyes these brilliant colours
reflected from the grating, and you have also seen them thrown upon the screen from a
grating placed in the lantern. Now with a grating of 17,000 lines per inch - a much
greater number than the other - you will see how much further from the central bright
space the first spectrum is; how much more this grating changes the direction, or
diffraction, of the beam of light. Here is the centre of the grating, and there is the
first spectrum. You will note that the violet light is least diffracted and the red light
is most diffracted. This diffraction of light first proved to us definitely the reality of
the undulatory theory of light.
You ask why does not light go round a corner as sound does. Light does go round a
corner in these diffraction spectrums; and it is shown going round a corner, since it
passes through these bars and is turned round an angle of thirty degrees. The phenomena of
light going round a corner seen by means of instruments adapted to show the result and to
measure the angles through which it is turned, is called the diffraction of light.
I can show you an instrument which will measure the wave - lengths of light. Without
proving the formula, let me tell it to you. A spirit - lamp with salt sprinkled on the
wick gives very nearly homogeneous light, that is to say, light of one wave - length, or
all of the same period. I have here a little grating which I take in my hand. I look
through this grating and see that candle before me. Close behind it you see a blackened
slip of wood with two white marks on it ten inches asunder. The line on which they are
marked is placed perpendicular to the line at which I shall go from it. When I look at
this salted spirit - lamp I see a series of spectrums of yellow light. As I am somewhat
short - sighted I am making my eye see with this eye - glass and the natural lenses of the
eye what a long - sighted person would make out without an eye - glass. On that screen you
saw a succession of spectrums. I now look direct at the candle and what do I see? I see a
succession of five or six brilliantly coloured spectrums on each side of the candle. But
when I look at the salted spirit - lamp, now I see ten spectrums on one side and ten on
the other, each of which is a monochromatic band of light.
I will measure the wave - length of the light thus. I walk away to a considerable
distance and look at the spirit - lamp and marks. I see a set of spectrums, The first
white line is exactly behind the flame. I want the first spectrum to the right of that
white line to fall exactly on the other white line, which is ten inches from the first. As
I walk away from it I see it is now very near it; it is now on it. Now the distance from
my eye is to be measured, and the problem is again to reduce feet to inches. The distance
from the spectrum of the flame to my eye is thirty - four feet nine inches. Mr. President,
how many inches is that? 417 inches, in round numbers 420 inches. Then we have the
proportion, as 420 is to 10 so is the length from bar to bar of the grating to the wave -
length of sodium light. That is to say as forty two is to one. The distance from bar to
bar is the four hundredth of a centimetre: therefore the forty - second part of the four
hundredth of a centimetre is the wave - length according to our simple, and easy and hasty
experiment. The true wave - length of sodium light, according to the most accurate
measurement, is about a 17,000th of a centimetre, which differs by scarcely more than one
per cent. from our result!
The only apparatus you see is this little grating - a piece of glass having a space
four - tents of an inch wide ruled with 400 fine lines. Any of you who will take the
trouble to buy one may measure the wave - length of a candle flame himself. I hope some of
you will be induced to make the experiment for yourselves.
If I put salt on the flame of a spirit - lamp, what do I see through this grating? I
see merely a sharply defined yellow light, constituting the spectrum of vaporised sodium,
while from the candle flame I see an exquisitely coloured spectrum, far more beautiful
than that I showed you on the screen. I see in fact a series of spectrums on the two sides
with the blue toward the candle flame and the red further out. I cannot get one definite
thing to measure from in the spectrum from the candle flame, as I can with the flame of a
spirit - lamp with the salt thrown on it, which gives as I have said a simple yellow
light. The highest blue light I see in the candle flame is now exactly on the line. Now
measure to my eye, it is forty - four feet four inches, or 532 inches. The length of this
wave then is the 532d part of the four hundredth of a centimetre which would be the
21,280th of a centimetre, say the 21,000th of a centimetre. Then measure for the red and
you will find something like the 11,000th for the lowest of the red light.
Lastly, how do we know the frequency of vibration?
Why, by the velocity of light. How do we know that? We know it in a number of different
ways, which I cannot explain now because time forbids, and I can now only tell you shortly
that the frequency of vibration for any particular ray is equal to the velocity of light
divided by the wave - length for that ray. The velocity of light is about 187,000 British
statute miles per second, but it is much better to take the kilometre - which is about six
tenths of a mile - for the unit, when we find the velocity is very accurately 300,000
kilometres, or 30,000,000,000 centimetres, per second. Take now the wave - length of
sodium light, as we have just measured it by means of the salted spirit - lamp, to be one
17,000th of a centimetre, and we find the frequency of vibration of the sodium light to be
510 million million per second. There, then, you have a calculation of the frequency from
a simple observation which you all can make for yourselves.
Lastly, I must tell you about the colour of the blue sky which is illustrated by this
spherule imbedded in an elastic solid (Fig. 121). I want to explain to you in two minutes
the mode of vibration. Take the simplest plane - polarised light. Here is a spherule which
is producing it in an elastic solid. Imagine the solid to extend miles horizontally and
miles up and down, and imagine this spherule to vibrate up and down. It is quite clear
that it will make transverse vibrations similarly in all horizontal directions. The plane
of polarisation is defined as a plane perpendicular to the line of vibration. Thus, light
produced by a molecule vibrating up and down, as this red glove in the jelly before you,
is polarised in a horizontal plane because the vibrations are vertical.
Here is another mode of vibration. Let me twist this spherule in the jelly as I am now
doing, and that will produce vibrations, also spreading out equally in all horizontal
directions. When I twist this glove round it draws the jelly round with it; twist it
rapidly back and the jelly flies back. By the inertia of the jelly the vibrations spread
in all directions and the lines of vibration are horizontal all through the jelly.
Everywhere, miles away that solid is placed in vibration. You do not see the vibrations,
but you must understand that they are there. If it flies back it makes vibration, and we
have waves of horizontal vibrations travelling out in all directions from the exciting
molecule.
I am now causing the red glove to vibrate to and fro horizontally. That will cause
vibrations to be produced which will be parallel to the line of motion at all places of
the plane perpendicular to the range of the exciting molecule. What makes the blue sky?
These are exactly the motions that make the blue light of the sky, which is due to
spherules in the luminiferous ether, but little modified by the air. Think of the sun near
the horizon, think of the light of the sun streaming through and giving you the azure blue
and violet overhead. Think first of any one particle and think of it moving in such a way
as to give horizontal and vertical vibrations and circular and elliptic vibrations.
You see the blue sky in high pressure steam blown into the air; you see it in the
experiment of Tyndall's blue sky in which a delicate condensation of vapour gives rise to
exactly the azure blue of the sky.
Now the motion of the luminiferous ether relatively to the spherule gives rise to the
same effect as would an opposite motion impressed upon the spherule quite independently by
an independent force. So you may think of the blue colour coming from the sky as being
produced by to and fro vibrations of matter in the air, which vibrates much as this little
glove vibrates imbedded in the jelly.
The result in a general way is this: The light coming from the blue sky is polarised in
a plane through the sun, but the blue light of the sky is complicated by a great number of
circumstances and one of them is this, that the air is illuminated not only by the sun but
by the earth. If we could get the earth covered by a black cloth then we could study the
polarised light of the sky with a simplicity which we cannot do now. There are, in nature,
reflections from the seas and rocks and hills and waters in an infinitely complicated
manner.
Let observers observe the blue sky not only in winter when the earth is covered with
snow, but in summer when it is covered with dark green foliage. This will help to unravel
the complicated phenomena in question. But the azure blue of the sky is light produced by
the reaction on the vibrating ether of little spherules of water, of perhaps a fifty
thousandth or a hundred thousandth of a centimetre diameter, or perhaps little motes, or
lumps, or crystals of common salt, or particles of dust, or germs of vegetable or animal
species wafted about in the air. Now what is the luminiferous ether? It is matter
prodigiously less dense than air - millions and millions and millions of times less dense
than air. We can form some sort of idea of its limitations. We believe it is a real thing,
with great rigidity in comparison with its density: it may be made to vibrate 400 million
million times per second; and yet be of such density as not to produce the slightest
resistance to any body going through it.
Going back to the illustration of the shoemakers' wax; if a cork will, in the course of
a year, push its way up through a plate of that wax when placed under water, and if a lead
bullet will penetrate downwards to the bottom, what is the law of the resistance? It
clearly depends on time. The cork slowly in the course of a year works its way up through
two inches of that substance; give it one or two thousand years to do it and the
resistance will be enormously less; thus the motion of a cork or bullet, at the rate of
one inch in 2,000 years, may be compared with that of the earth, moving at the rate of six
times ninety - three million miles a year, or nineteen miles per second, through the
luminiferous ether; but when we can have actually before us a thing elastic like jelly and
yielding like pitch, surely we have a large and solid ground for our faith in the
speculative hypothesis of an elastic luminiferous ether, which constitutes the wave theory
of light.
Source:
Scientific papers: physics, chemistry, astronomy, geology, with introductions,
notes and illustrations. New York, P. F. Collier & son [c1910] Harvard
classics ; no.XXX.
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© Paul Halsall, August 1998
