CHRISTIAN HUYGENS
If for nothing else, the world is indebted to the man who
invented the pendulum clock, Christian Huygens (1629-1695), of
physicist. Huygens was the descendant of a noble and
distinguished family, his father, Sir Constantine Huygens, being
a well-known poet and diplomatist. Early in life young Huygens
began his career in the legal profession, completing his
education
in the juridical school at
mathematics soon led him to neglect his legal studies, and his
aptitude for scientific researches was so marked that Descartes
predicted great things of him even while he was a mere tyro in
the field of scientific investigation.
One of his first endeavors in science was to attempt an
improvement of the telescope. Reflecting upon the process of
making lenses then in vogue, young Huygens and his brother
Constantine attempted a new method of grinding and polishing,
whereby they overcame a great deal of the spherical and chromatic
aberration. With this new telescope a much clearer field of
vision was obtained, so much so that Huygens was able to detect 11411y2418l ,
among other things, a hitherto unknown satellite of Saturn. It
was these astronomical researches that led him to apply the
pendulum to regulate the movements of clocks. The need for some
more exact method of measuring time in his observations of the
stars was keenly felt by the young astronomer, and after several
experiments along different lines, Huygens hit upon the use of a
swinging weight; and in 1656 made his invention of the pendulum
clock. The year following, his clock was presented to the
states-general. Accuracy as to time is absolutely essential in
astronomy, but until the invention of Huygens's clock there was
no precise, nor even approximately precise, means of measuring
short intervals.
Huygens was one of the first to adapt the micrometer to the
telescope--a mechanical device on which all the nice
determination of minute distances depends. He also took up the
controversy against Hooke as to the superiority of telescopic
over plain sights to quadrants, Hooke contending in favor of the
plain. In this controversy, the subject of which attracted wide
attention, Huygens was completely victorious; and Hooke, being
unable to refute Huygens's arguments, exhibited such irritability
that he increased his already general unpopularity. All of the
arguments for and against the telescope sight are too numerous to
be given here. In contending in its favor Huygens pointed out
that the unaided eye is unable to appreciate an angular space in
the sky less than about thirty seconds. Even in the best quadrant
with a plain sight, therefore, the altitude must be uncertain by
that quantity. If in place of the plain sight a telescope is
substituted, even if it magnify only thirty times, it will enable
the observer to fix the position to one second, with
progressively increased accuracy as the magnifying power of the
telescope is increased. This was only one of the many telling
arguments advanced by Huygens.
In the field of optics, also, Huygens has added considerably to
science, and his work, Dioptrics, is said to have been a favorite
book with Newton. During the later part of his life, however,
Huygens again devoted himself to inventing and constructing
telescopes, grinding the lenses, and devising, if not actually
making, the frame for holding them. These telescopes were of
enormous lengths, three of his object-glasses, now in possession
of the Royal Society, being of 123, 180, and 210 feet focal
length respectively. Such instruments, if constructed in the
ordinary form of the long tube, were very unmanageable, and to
obviate this Huygens adopted the plan of dispensing with the tube
altogether, mounting his lenses on long poles manipulated by
machinery. Even these were unwieldy enough, but the difficulties
of manipulation were fully compensated by the results obtained.
It had been discovered, among other things, that in oblique
refraction light is separated into colors. Therefore, any small
portion of the convex lens of the telescope, being a prism, the
rays proceed to the focus, separated into prismatic colors, which
make the image thus formed edged with a fringe of color and
indistinct. But, fortunately for the early telescope makers, the
degree of this aberration is independent of the focal length of
the lens; so that, by increasing this focal length and using the
appropriate eye-piece, the image can be greatly magnified, while
the fringe of colors remains about the same as when a less
powerful lens is used. Hence the advantage of Huygens's long
telescope. He did not confine his efforts to simply lengthening
the focal length of his telescopes, however, but also added to
their efficiency by inventing an almost perfect achromatic
eye-piece.
In 1663 he was elected a fellow of the Royal Society of London,
and in 1669 he gave to that body a concise statement of the laws
governing the collision of elastic bodies. Although the same
views had been given by Wallis and Wren a few weeks earlier,
there is no doubt that Huygens's views were reached
independently; and it is probable that he had arrived at his
conclusions several years before. In the Philosophical
Transactions for 1669 it is recorded that the society, being
interested in the laws of the principles of motion, a request was
made that M. Huygens, Dr. Wallis, and Sir Christopher Wren submit
their views on the subject. Wallis submitted his paper first,
November 15, 1668. A month later, December 17th, Wren imparted to
the society his laws as to the nature of the collision of bodies.
And a few days later, January 5, 1669, Huygens sent in his "Rules
Concerning the Motion of Bodies after Mutual Impulse." Although
Huygens's report was received last, he was anticipated by such a
brief space of time, and his views are so clearly stated--on the
whole rather more so than those of the other two--that we give
them in part here:
"1. If a hard body should strike against a body equally hard at
rest, after contact the former will rest and the latter acquire a
velocity equal to that of the moving body.
"2. But if that other equal body be likewise in motion, and
moving in the same direction, after contact they will move with
reciprocal velocities.
"3. A body, however great, is moved by a body however small
impelled with any velocity whatsoever.
"5. The quantity of motion of two bodies may be either increased
or diminished by their shock; but the same quantity towards the
same part remains, after subtracting the quantity of the contrary
motion.
"6. The sum of the products arising from multiplying the mass of
any hard body into the squares of its velocity is the same both
before and after the stroke.
"7. A hard body at rest will receive a greater quantity of motion
from another hard body, either greater or less than itself, by
the interposition of any third body of a mean quantity, than if
it was immediately struck by the body itself; and if the
interposing body be a mean proportional between the other two,
its action upon the quiescent body will be the greatest of
all."[10]
This was only one of several interesting and important
communications sent to the Royal Society during his lifetime. One
of these was a report on what he calls "Pneumatical Experiments."
"Upon including in a vacuum an insect resembling a beetle, but
somewhat larger," he says, "when it seemed to be dead, the air
was readmitted, and soon after it revived; putting it again in
the vacuum, and leaving it for an hour, after which the air was
readmitted, it was observed that the insect required a longer
time to recover; including it the third time for two days, after
which the air was admitted, it was ten hours before it began to
stir; but, putting it in a fourth time, for eight days, it never
afterwards recovered.... Several birds, rats, mice, rabbits, and
cats were killed in a vacuum, but if the air was admitted before
the engine was quite exhausted some of them would recover; yet
none revived that had been in a perfect vacuum.... Upon putting
the weight of eighteen grains of powder with a gauge into a
receiver that held several pounds of water, and firing the
powder, it raised the mercury an inch and a half; from which it
appears that there is one-fifth of air in gunpowder, upon the
supposition that air is about one thousand times lighter than
water; for in this experiment the mercury rose to the eighteenth
part of the height at which the air commonly sustains it, and
consequently the weight of eighteen grains of powder yielded air
enough to fill the eighteenth part of a receiver that contained
seven pounds of water; now this eighteenth part contains
forty-nine drachms of water; wherefore the air, that takes up an
equal space, being a thousand times lighter, weighs
one-thousandth part of forty-nine drachms, which is more than
three grains and a half; it follows, therefore, that the weight
of eighteen grains of powder contains more than three and a half
of air, which is about one-fifth of eighteen grains...."
From 1665 to 1681, accepting the tempting offer made him through
Colbert, by Louis XIV., Huygens pursued his studies at the
Bibliotheque du Roi as a resident of France. Here he published
his Horologium Oscillatorium, dedicated to the king, containing,
among other things, his solution of the problem of the "centre of
oscillation." This in itself was an important step in the history
of mechanics. Assuming as true that the centre of gravity of any
number of interdependent bodies cannot rise higher than the point
from which it falls, he reached correct conclusions as to the
general principle of the conservation of vis viva, although he
did not actually prove his conclusions. This was the first
attempt to deal with the dynamics of a system. In this work,
also, was the true determination of the relation between the
length of a pendulum and the time of its oscillation.
In 1681 he returned to Holland, influenced, it is believed, by
the attitude that was being taken in France against his religion.
Here he continued his investigations, built his immense
telescopes, and, among other things, discovered "polarization,"
which is recorded in Traite de la Lumiere, published at Leyden in
1690. Five years later he died, bequeathing his manuscripts to
the University of Leyden. It is interesting to note that he never
accepted Newton's theory of gravitation as a universal property
of matter.
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