Introduction.
In Cornu's elaborate memoir upon the determination of the velocity of
light, several objections are made to the plan followed by Foucault, which
will be considered in the latter part of this work. It may, however, be
stated that the most important among these was that the deflection was too
small to be measured with the required degree of accuracy. In order to
employ this method, therefore, it was absolutely necessary that the
deflection should be increased.
In November, 1877, a modification of Foucault's arrangement suggested
itself, by which this result could be accomplished. Between this time and
March of the following year a number of preliminary experiments were
performed in order to familiarize myself with the optical arrangements.
The first experiment tried with the revolving mirror produced a deflection
considerably greater than that obtained by Foucault. Thus far the only
apparatus used was such as could be adapted from the apparatus in the
laboratory of the Naval Academy.
At the expense of $10 a revolving mirror was made, which could execute 128
turns per second. The apparatus was installed in May, 1878, at the
laboratory. The distance used was 500 feet, and the deflection was about
twenty times that obtained by Foucault.[1]
[Footnote 1: See Proc. Am. Assoc. Adv. Science, Saint Louis meeting.]
These experiments, made with very crude apparatus and under great
difficulties, gave the following table of results for the velocity of
light in miles per second:
186730
188820
186330
185330
187900
184500
186770
185000
185800
187940
------
Mean 186500 ± 300 miles per second,
or 300140 kilometers per second.
In the following July the sum of $2,000 was placed at my disposal by a
private gentleman for carrying out these experiments on a large scale.
Before ordering any of the instruments, however, it was necessary to find
whether or not it was practicable to use a large distance. With a distance
(between the revolving and the fixed mirror) of 500 feet, in the
preliminary experiments, the field of light in the eye-piece was somewhat
limited, and there was considerable indistinctness in the image, due to
atmospheric disturbances.
Accordingly, the same lens (39 feet focus) was employed, being placed,
together with the other pieces of apparatus, along the north sea-wall of
the Academy grounds, the distance being about 2,000 feet. The image of the
slit, at noon, was so confused as not to be recognizable, but toward
sunset it became clear and steady, and measurements were made of its
position, which agreed within one one-hundredth of a millimeter. It was
thus demonstrated that with this distance and a deflection of 100
millimeters this measurement could be made within the ten-thousandth part.
In order to obtain this deflection, it was sufficient to make the mirror
revolve 250 times per second and to use a "radius" of about 30 feet. In
order to use this large radius (distance from slit to revolving mirror),
it was necessary that the mirror should be large and optically true; also,
that the lens should be large and of great focal length. Accordingly the
mirror was made 1¼ inches in diameter, and a new lens, 8 inches in
diameter, with a focal length of 150 feet was procured.
In January, 1879, an observation was taken, using the old lens, the mirror
making 128 turns per second. The deflection was about 43 millimeters. The
micrometer eye-piece used was substantially the same as Foucault's, except
that part of the inclined plate of glass was silvered, thus securing a
much greater quantity of light. The deflection having reached 43
millimeters, the inclined plate of glass could be dispensed with, the
light going past the observer's head through the slit, and returning 43
millimeters to the left of the slit, where it could be easily observed.
Thus the micrometer eye-piece is much simplified, and many possible
sources of error are removed.
The field was quite limited, the diameter being, in fact, but little
greater than the width of the slit. This would have proved a most serious
objection to the new arrangement. With the new lens, however, this
difficulty disappeared, the field being about twenty times the width of
the slit. It was expected that, with the new lens, the image would be less
distinct; but the difference, if any, was small, and was fully compensated
by the greater size of the field.
The first observation with the new lens was made January 30, 1879. The
deflection was 70 millimeters. The image was sufficiently bright to be
observed without the slightest effort. The first observation with the new
micrometer eye-piece was made April 2, the deflection being 115
millimeters.
The first of the final series of observations was made on June 5. All the
observations previous to this, thirty sets in all, were rejected. After
this time, no set of observations nor any single observation was omitted.
Theory of New Method.
[Illustration: FIG. 1.]
Let S, Fig. 1, be a slit, through which light passes, falling on R, a
mirror free to rotate about an axis at right angles to the plane of the
paper; L, a lens of great focal length, upon which the light falls which
is reflected from R. Let M be a plane mirror whose surface is
perpendicular to the line R, M, passing through the centers of R, L, and
M, respectively. If L be so placed that an image of S is formed on the
surface of M, then, this image acting as the object, its image will be
formed at S, and will coincide, point for point, with S.
If, now, R be turned about the axis, so long as the light falls upon the
lens, an image of the slit will still be formed on the surface of the
mirror, though on a different part, and as long as the returning light
falls on the lens an image of this image will be formed at S,
notwithstanding the change of position of the first image at M. This
result, namely, the production of a stationary image of an image in
motion, is absolutely necessary in this method of experiment. It was first
accomplished by Foucault, and in a manner differing apparently but little
from the foregoing.
[Illustration: FIG. 2.]
In his experiments L, Fig. 2, served simply to form the image of S at M,
and M, the returning mirror, was spherical, the center coinciding with the
axis of R. The lens L was placed as near as possible to R. The light
forming the return image lasts, in this case, while the first image is
sweeping over the face of the mirror, M. Hence, the greater the distance
RM, the larger must be the mirror in order that the same amount of light
may be preserved, and its dimensions would soon become inordinate. The
difficulty was partly met by Foucault, by using five concave reflectors
instead of one, but even then the greatest distance he found it
practicable to use was only 20 meters.
Returning to Fig. 1, suppose that R is in the principal focus of the lens
L; then, if the plane mirror M have the same diameter as the lens, the
first, or moving image, will remain upon M as long as the axis of the
pencil of light remains on the lens, and _this will be the case no matter
what the distance may be_.
When the rotation of the mirror R becomes sufficiently rapid, then the
flashes of light which produce the second or stationary image become
blended, so that the image appears to be continuous. But now it no longer
coincides with the slit, but is _deflected_ in the direction of rotation,
and through twice the angular distance described by the mirror, during
the time required for light to travel twice the distance between the
mirrors. This displacement is measured by the tangent of the arc it
subtends. To make this as large as possible, the distance between the
mirrors, the radius, and the speed of rotation should be made as great as
possible.
The second condition conflicts with the first, for the radius is the
difference between the focal length for parallel rays, and that for rays
at the distance of the fixed mirror. The greater the distance, therefore,
the smaller will be the radius.
There are two ways of solving the difficulty: first, by using a lens of
great focal length; and secondly, by placing the revolving mirror within
the principal focus of the lens. Both means were employed. The focal
length of the lens was 150 feet, and the mirror was placed about 15 feet
within the principal focus. A limit is soon reached, however, for the
quantity of light received diminishes very rapidly as the revolving mirror
approaches the lens.
Arrangement and Description of Apparatus.
Site and Plan.
The site selected for the experiments was a clear, almost level, stretch
along the north sea-wall of the Naval Academy. A frame building was
erected at the western end of the line, a plan of which is represented in
Fig. 3.
[Illustration: FIG. 3.]
The building was 45 feet long and 14 feet wide, and raised so that the
line along which the light traveled was about 11 feet above the ground. A
heliostat at H reflected the sun's rays through the slit at S to the
revolving mirror R, thence through a hole in the shutter, through the
lens, and to the distant mirror.
The Heliostat.
The heliostat was one kindly furnished by Dr. Woodward, of the Army
Medical Museum, and was a modification of Foucault's form, designed by
Keith. It was found to be accurate and easy to adjust. The light was
reflected from the heliostat to a plane mirror, M, Fig. 3, so that the
former need not be disturbed after being once adjusted.
The Revolving Mirror.
The revolving mirror was made by Fauth & Co., of Washington. It consists
of a cast-iron frame resting on three leveling screws, one of which was
connected by cords to the table at S, Fig. 3, so that the mirror could be
inclined forward or backward while making the observations.
[Illustration: FIG. 4.]
Two binding screws, S, S, Fig. 4, terminating in hardened steel conical
sockets, hold the revolving part. This consists of a steel axle, X, Y,
Figs. 4 and 5, the pivots being conical and hardened. The axle expands
into a ring at R, which holds the mirror M. The latter was a disc of plane
glass, made by Alvan Clark & Sons, about 1¼ inch in diameter and 0.2 inch
thick. It was silvered on one side only, the reflection taking place from
the outer or front surface. A species of turbine wheel, T, is held on the
axle by friction. This wheel has six openings for the escape of air; a
section of one of them is represented in Fig 6.
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
Adjustment of the Revolving Mirror.
The air entering on one side at O, Fig. 5, acquires a rotary motion in the
box B, B, carrying the wheel with it, and this motion is assisted by the
reaction of the air in escaping. The disc C serves the purpose of bringing
the center of gravity in the axis of rotation. This was done, following
Foucault's plan, by allowing the pivots to rest on two inclined planes of
glass, allowing the arrangement to come to rest, and filing away the
lowest part of the disc; trying again, and so on, till it would rest in
indifferent equilibrium. The part corresponding to C, in Foucault's
apparatus, was furnished with three vertical screws, by moving which the
axis of figure was brought into coincidence with the axis of rotation.
This adjustment was very troublesome. Fortunately, in this apparatus it
was found to be unnecessary.
When the adjustment is perfect the apparatus revolves without giving any
sound, and when this is accomplished, the motion is regular and the speed
great. A slight deviation causes a sound due to the rattling of the pivots
in the sockets, the speed is very much diminished, and the pivots begin to
wear. In Foucault's apparatus oil was furnished to the pivots, through
small holes running through the screws, by pressure of a column of
mercury. In this apparatus it was found sufficient to touch the pivots
occasionally with a drop of oil.
[Illustration: FIG. 7.]
Fig. 7 is a view of the turbine, box, and supply-tube, from above. The
quantity of air entering could be regulated by a valve to which was
attached a cord leading to the observer's table.
The instrument was mounted on a brick pier.
The Micrometer.
[Illustration: FIG. 8.]
The apparatus for measuring the deflection was made by Grunow, of New
York.
This instrument is shown in perspective in Fig. 8, and in plan by Fig. 9.
The adjustable slit S is clamped to the frame F. A long millimeter-screw,
not shown in Fig. 8, terminating in the divided head D, moves the carriage
C, which supports the eye-piece E. The frame is furnished with a brass
scale at F for counting revolutions, the head counting hundredths. The
eye-piece consists of a single achromatic lens, whose focal length is
about two inches. At its focus, in H, and in nearly the same plane as the
face of the slit, is a single vertical silk fiber. The apparatus is
furnished with a standard with rack and pinion, and the base furnished
with leveling screws.
Manner of Using the Micrometer.
In measuring the deflection, the eye-piece is moved till the cross-hair
bisects the slit, and the reading of the scale and divided head gives the
position. This measurement need not be repeated unless the position or
width of the slit is changed. Then the eye-piece is moved till the
cross-hair bisects the deflected image of the slit; the reading of scale
and head are again taken, and the difference in readings gives the
deflection. The screw was found to have no lost motion, so that readings
could be taken with the screw turned in either direction.
Measurement of Speed of Rotation.
To measure the speed of rotation, a tuning-fork, bearing on one prong a
steel mirror, was used. This was kept in vibration by a current of
electricity from five "gravity" cells. The fork was so placed that the
light from the revolving mirror was reflected to a piece of plane glass,
in front of the lens of the eye-piece of the micrometer, inclined at an
angle of 45°, and thence to the eye. When fork and revolving mirror are
both at rest, an image of the revolving mirror is seen. When the fork
vibrates, this image is drawn out into a band of light.
When the mirror commences to revolve, this band breaks up into a number of
moving images of the mirror; and when, finally, the mirror makes as many
turns as the fork makes vibrations, these images are reduced to one, which
is stationary. This is also the case when the number of turns is a
submultiple. When it is a multiple or simple ratio, the only difference is
that there are more images. Hence, to make the mirror execute a certain
number of turns, it is simply necessary to pull the cord attached to the
valve to the right or left till the images of the revolving mirror come to
rest.
The electric fork made about 128 vibrations per second. No dependence was
placed upon this rate, however, but at each set of observations it is
compared with a standard Ut₃ fork, the temperature being noted at the
same time. In making the comparison the sound-beats produced by the forks
were counted for 60 seconds. It is interesting to note that the electric
fork, as long as it remained untouched and at the same temperature, did
not change its rate more than one or two hundredths vibrations per second.
[Illustration: FIG. 9.]
The Observer's Table.
Fig. 9 Represents The Table At Which The Observer Sits. The Light From The
Heliostat Passes Through The Slit At S, Goes To The Revolving Mirror, &c.,
And, On Its Return, Forms An Image Of The Slit At D, Which Is Observed
Through The Eye-piece. E Represents The Electric Fork (the Prongs Being
Vertical) Bearing The Steel Mirror M. K Is The Standard Fork On Its
Resonator. C Is The Cord Attached To The Valve Supplying Air To The
Turbine.
The Lens.
The lens was made by Alvan Clark & Sons. It was 8 inches in diameter;
focal length, 150 feet; not achromatic. It was mounted in a wooden frame,
which was placed on a support moving on a slide, about 16 feet long,
placed about 80 feet from the building. As the diameter of the lens was so
small in comparison with its focal length, its want of achromatism was
inappreciable. For the same reason, the effect of "parallax" (due to want
of coincidence in the plane of the image with that of the silk fiber in
the eye-piece) was too small to be noticed.
The Fixed Mirror.
The fixed mirror was one of those used in taking photographs of the
transit of Venus. It was about 7 inches in diameter, mounted in a brass
frame capable of adjustment in a vertical and a horizontal plane by screw
motion. Being wedge-shaped, it had to be silvered on the front surface. To
facilitate adjustment, a small telescope furnished with cross-hairs was
attached to the mirror by a universal joint. The heavy frame was mounted
on a brick pier, and the whole surrounded by a wooden case to protect it
from the sun.
Adjustment of the Fixed Mirror.
The adjustment was effected as follows: A theodolite was placed at about
100 feet in front of the mirror, and the latter was moved about by the
screws till the observer at the theodolite saw the image of his telescope
reflected in the center of the mirror. Then the telescope attached to the
mirror was pointed (without moving the mirror itself) at a mark on a piece
of card-board attached to the theodolite. Thus the line of collimation of
the telescope was placed at right angles to the surface of the mirror. The
theodolite was then moved to 1,000 feet, and, if found necessary, the
adjustment was repeated. Then the mirror was moved by the screws till its
telescope pointed at the hole in the shutter of the building. The
adjustment was completed by moving the mirror, by signals, till the
observer, looking through the hole in the shutter, through a good
spy-glass, saw the image of the spy-glass reflected centrally in the
mirror.
The whole operation was completed in a little over an hour.
Notwithstanding the wooden case about the pier, the mirror would change
its position between morning and evening; so that the last adjustment had
to be repeated before every series of experiments.
Apparatus for Supplying and Regulating the Blast of Air.
Fig. 10 represents a plan of the lower floor of the building. E is a
three-horse power Lovegrove engine and boiler, resting on a stone
foundation; B, a small Roots' blower; G, an automatic regulator. From this
the air goes to a delivery-pipe, up through the floor, and to the turbine.
The engine made about 4 turns per second and the blower about 15. At this
speed the pressure of the air was about half a pound per square inch.
[Illustration: FIG. 10.]
The regulator, Fig. 11, consists of a strong bellows supporting a weight
of 370 pounds, partly counterpoised by 80 pounds in order to prevent the
bellows from sagging. When the pressure of air from the blower exceeds the
weight, the bellows commences to rise, and, in so doing, closes the
valve V.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
This arrangement was found in practice to be insufficient, and the
following addition was made: A valve was placed at P, and the pipe was
tapped a little farther on, and a rubber tube led to a water-gauge, Fig
12. The column of water in the smaller tube is depressed, and, when it
reaches the horizontal part of the tube, the slightest variation of
pressure sends the column from one end to the other. This is checked by an
assistant at the valve; so that the column of water is kept at about the
same place, and the pressure thus rendered very nearly constant. The
result was satisfactory, though not in the degree anticipated. It was
possible to keep the mirror at a constant speed for three or four seconds
at a time, and this was sufficient for an observation. Still it would have
been more convenient to keep it so for a longer time.
I am inclined to think that the variations were due to changes in the
friction of the pivots rather than to changes of pressure of the blast of
air.
It may be mentioned that the test of uniformity was very delicate, as a
change of speed of one or two hundredths of a turn per second could easily
be detected.
Method Followed in Experiment.
It was found that the only time during the day when the atmosphere was
sufficiently quiet to get a distinct image was during the hour after
sunrise, or during the hour before sunset. At other times the image was
"boiling" so as not to be recognizable. In one experiment the electric
light was used at night, but the image was no more distinct than at
sunset, and the light was not steady.
The method followed in experiment was as follows: The fire was started
half an hour before, and by the time everything was ready the gauge would
show 40 or 50 pounds of steam. The mirror was adjusted by signals, as
before described. The heliostat was placed and adjusted. The revolving
mirror was inclined to the right or left, so that the _direct_ reflection
of light from the slit, which otherwise would flash into the eye-piece at
every revolution, fell either above or below the eye-piece.[2]
[Footnote 2: Otherwise this light would overpower that which forms the
image to be observed. As far as I am aware, Foucault does not speak of
this difficulty. If he allowed this light to interfere with the
brightness of the image, he neglected a most obvious advantage. If he
did incline the axis of the mirror to the right or left, he makes no
allowance for the error thus introduced.]
The revolving mirror was then adjusted by being moved about, and inclined
forward and backward, till the light was seen reflected back from the
distant mirror. This light was easily seen through the coat of silver on
the mirror.
The distance between the front face of the revolving mirror and the
cross-hair of the eye-piece was then measured by stretching from the one
to the other a steel tape, making the drop of the catenary about an inch,
as then the error caused by the stretch of the tape and that due to the
curve just counterbalance each other.
The position of the slit, if not determined before, was then found as
before described. The electric fork was started, the temperature noted,
and the sound-beats between it and the standard fork counted for 60
seconds. This was repeated two or three times before every set of
observations.
The eye-piece of the micrometer was then set approximately[3] and the
revolving mirror started. If the image did not appear, the mirror was
inclined forward or backward till it came in sight.
[Footnote 3: The deflection being measured by its tangent, it was
necessary that the scale should be at right angles to the radius (the
radius drawn from the mirror to one or the other end of that part of
the scale which represents this tangent). This was done by setting the
eye-piece approximately to the expected deflection, and turning the
whole micrometer about a vertical axis till the cross-hair bisected the
circular field of light reflected from the revolving mirror. The axis
of the eye-piece being at right angles to the scale, the latter would
be at right angles to radius drawn to the cross-hair.]
The cord connected with the valve was pulled right or left till the images
of the revolving mirror, represented by the two bright round spots to the
left of the cross-hair, came to rest. Then the screw was turned till the
cross-hair bisected the deflected image of the slit. This was repeated
till ten observations were taken, when the mirror was stopped, temperature
noted, and beats counted. This was called a set of observations. Usually
five such sets were taken morning and evening.
[Illustration: FIG. 13.]
Fig. 13 represents the appearance of the image of the slit as seen in the
eye-piece magnified about five times.
Determination of The Constants.
Comparison of the Steel Tape with the Standard Yard.
The steel tape used was one of Chesterman's, 100 feet long. It was
compared with Wurdeman's copy of the standard yard, as follows:
Temperature was 55° Fahr.
The standard yard was brought under the microscopes of the comparator; the
cross-hair of the unmarked microscope was made to bisect the division
marked o, and the cross-hair of the microscope, marked I, was made to
bisect the division marked 36. The reading of microscope I was taken, and
the other microscope was not touched during the experiment. The standard
was then removed and the steel tape brought under the microscopes and
moved along till the division marked 0.1 (feet) was bisected by the
cross-hair of the unmarked microscope. The screw of microscope I was then
turned till its cross-hair bisected the division marked 3.1 (feet), and
the reading of the screw taken. The difference between the original
reading and that of each measurement was noted, care being taken to regard
the direction in which the screw was turned, and this gave the difference
in length between the standard and each succesive portion of the steel
tape in terms of turns of the micrometer-screw.
To find the value of one turn, the cross-hair was moved over a millimeter
scale, and the following were the values obtained: