INSTRUMENTS
OF PRECISION IN THE AGE OF
During the Newtonian epoch there were numerous important
inventions of scientific instruments, as well as many
improvements made upon the older ones. Some of these discoveries
have been referred to briefly in other places, but their
importance in promoting scientific investigation warrants a
fuller treatment of some of the more significant.
Many of the errors that had arisen in various scientific
calculations before the seventeenth century may be ascribed to
the crudeness and inaccuracy in the construction of most
scientific instruments. Scientists had not as yet learned that an
approach to absolute accuracy was necessary in every
investigation in the field of science, and that such accuracy
must be extended to the construct 23423f510x ion of the instruments used in
these investigations and observations. In astronomy it is obvious
that instruments of delicate exactness are most essential; yet
Tycho Brahe, who lived in the sixteenth century, is credited with
being the first astronomer whose instruments show extreme care in
construction.
It seems practically settled that the first telescope was
invented in Holland in 1608; but three men, Hans Lippershey,
James Metius, and Zacharias Jansen, have been given the credit of
the invention at different times. It would seem from certain
papers, now in the library of the University of Leyden, and
included in Huygens's papers, that Lippershey was probably the
first to invent a telescope and to describe his invention. The
story is told that Lippershey, who was a spectacle-maker,
stumbled by accident upon the discovery that when two lenses are
held at a certain distance apart, objects at a distance appear
nearer and larger. Having made this discovery, be fitted two
lenses with a tube so as to maintain them at the proper distance,
and thus constructed the first telescope.
It was Galileo, however, as referred to in a preceding chapter,
who first constructed a telescope based on his knowledge of the
laws of refraction. In 1609, having heard that an instrument had
been invented, consisting of two lenses fixed in a tube, whereby
objects were made to appear larger and nearer, he set about
constructing such an instrument that should follow out the known
effects of refraction. His first telescope, made of two lenses
fixed in a lead pipe, was soon followed by others of improved
types, Galileo devoting much time and labor to perfecting lenses
and correcting errors. In fact, his work in developing the
instrument was so important that the telescope came gradually to
be known as the "Galilean telescope."
In the construction of his telescope Galileo made use of a convex
and a concave lens; but shortly after this Kepler invented an
instrument in which both the lenses used were convex. This
telescope gave a much larger field of view than the Galilean
telescope, but did not give as clear an image, and in consequence
did not come into general use until the middle of the seventeenth
century. The first powerful telescope of this type was made by
Huygens and his brother. It was of twelve feet focal length, and
enabled Huygens to discover a new satellite of Saturn, and to
determine also the true explanation of Saturn's ring.
It was Huygens, together with Malvasia and Auzout, who first
applied the micrometer to the telescope, although the inventor of
the first micrometer was William Gascoigne, of Yorkshire, about
1636. The micrometer as used in telescopes enables the observer
to measure accurately small angular distances. Before the
invention of the telescope such measurements were limited to the
angle that could be distinguished by the naked eye, and were, of
course, only approximately accurate. Even very careful observers,
such as Tycho Brahe, were able to obtain only fairly accurate
results. But by applying Gascoigne's invention to the telescope
almost absolute accuracy became at once possible. The principle
of Gascoigne's micrometer was that of two pointers lying
parallel, and in this position pointing to zero. These were
arranged so that the turning of a single screw separated or
approximated them at will, and the angle thus formed could be
determined with absolute accuracy.
Huygens's micrometer was a slip of metal of variable breadth
inserted at the focus of the telescope. By observing at what
point this exactly covered an object under examination, and
knowing the focal length of the telescope and the width of the
metal, he could then deduce the apparent angular breadth of the
object. Huygens discovered also that an object placed in the
common focus of the two lenses of a Kepler telescope appears
distinct and clearly defined. The micrometers of Malvasia, and
later of Auzout and Picard, are the development of this
discovery. Malvasia's micrometer, which he described in 1662,
consisted of fine silver wires placed at right-angles at the
focus of his telescope.
As telescopes increased in power, however, it was found that even
the finest wire, or silk filaments, were much too thick for
astronomical observations, as they obliterated the image, and so,
finally, the spider-web came into use and is still used in
micrometers and other similar instruments. Before that time,
however, the fine crossed wires had revolutionized astronomical
observations. "We may judge how great was the improvement which
these contrivances introduced into the art of observing," says
Whewell, "by finding that Hevelius refused to adopt them because
they would make all the old observations of no value. He had
spent a laborious and active life in the exercise of the old
methods, and could not bear to think that all the treasures which
he had accumulated had lost their worth by the discovery of a new
mine of richer ones."[1]
Until the time of Newton, all the telescopes in use were either
of the Galilean or Keplerian type, that is, refractors. But about
the year 1670 Newton constructed his first reflecting telescope,
which was greatly superior to, although much smaller than, the
telescopes then in use. He was led to this invention by his
experiments with light and colors. In 1671 he presented to the
Royal Society a second and somewhat larger telescope, which he
had made; and this type of instrument was little improved upon
until the introduction of the achromatic telescope, invented by
Chester Moor Hall in 1733.
As is generally known, the element of accurate measurements of
time plays an important part in the measurements of the movements
of the heavenly bodies. In fact, one was scarcely possible
without the other, and as it happened it was the same man,
Huygens, who perfected Kepler's telescope and invented the
pendulum clock. The general idea had been suggested by Galileo;
or, better perhaps, the equal time occupied by the successive
oscillations of the pendulum had been noted by him. He had not
been able, however, to put this discovery to practical account.
But in 1656 Huygens invented the necessary machinery for
maintaining the motion of the pendulum and perfected several
accurate clocks. These clocks were of invaluable assistance to
the astronomers, affording as they did a means of keeping time
"more accurate than the sun itself." When Picard had corrected
the variation caused by heat and cold acting upon the pendulum
rod by combining metals of different degrees of expansibility, a
high degree of accuracy was possible.
But while the pendulum clock was an unequalled stationary
time-piece, it was useless in such unstable situations as, for
example, on shipboard. But here again Huygens played a prominent
part by first applying the coiled balance-spring for regulating
watches and marine clocks. The idea of applying a spring to the
balance-wheel was not original with Huygens, however, as it had
been first conceived by Robert Hooke; but Huygens's application
made practical Hooke's idea. In England the importance of
securing accurate watches or marine clocks was so fully
appreciated that a reward of L20,000 sterling was offered by
Parliament as a stimulus to the inventor of such a time-piece.
The immediate incentive for this offer was the obvious fact that
with such an instrument the determination of the longitude of
places would be much simplified. Encouraged by these offers, a
certain
carpenter named
subject of watch-making, and, after many years of labor, in 1758
produced a spring time-keeper which, during a sea-voyage
occupying one hundred and sixty-one days, varied only one minute
and five seconds. This gained for Harrison a reward Of L5000
sterling at once, and a little later L10,000 more, from
Parliament.
While inventors were busy with the problem of accurate
chronometers, however, another instrument for taking longitude at
sea had been invented. This was the reflecting quadrant, or
sextant, as the improved instrument is now called, invented by
John Hadley in 1731, and independently by Thomas Godfrey, a poor
glazier of Philadelphia, in 1730. Godfrey's invention, which was
constructed on the same principle as that of the Hadley
instrument, was not generally recognized until two years after
Hadley's discovery, although the instrument was finished and
actually in use on a sea-voyage some months before Hadley
reported his invention. The principle of the sextant, however,
seems to have been known to Newton, who constructed an instrument
not very unlike that of Hadley; but this invention was lost sight
of until several years after the philosopher's death and some
time after Hadley's invention.
The introduction of the sextant greatly simplified taking
reckonings at sea as well as facilitating taking the correct
longitude of distant places. Before that time the mariner was
obliged to depend upon his compass, a cross-staff, or an
astrolabe, a table of the sun's declination and a correction for
the altitude of the polestar, and very inadequate and incorrect
charts. Such were the instruments used by Columbus and Vasco da
Gama and their immediate successors.
During the Newtonian period the microscopes generally in use were
those constructed of simple lenses, for although compound
microscopes were known, the difficulties of correcting aberration
had not been surmounted, and a much clearer field was given by
the simple instrument. The results obtained by the use of such
instruments, however, were very satisfactory in many ways. By
referring to certain plates in this volume, which reproduce
illustrations from Robert Hooke's work on the microscope, it will
be seen that quite a high degree of effectiveness had been
attained. And it should be recalled that Antony von Leeuwenboek,
whose death took place shortly before Newton's, had discovered
such micro-organisms as bacteria, had seen the blood corpuscles
in circulation, and examined and described other microscopic
structures of the body.
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