10. INSTRUMENTS OF PRECISION—STATE OF THE SOLAR SYSTEM.
Having now traced the progress of physical astronomy up to the time
when very striking proofs of the universality of the law of gravitation
convinced the most sceptical, it must still be borne in mind that,
while gravitation is certainly the principal force governing the
motions of the heavenly bodies, there may yet be a resisting medium in
space, and there may be electric and magnetic forces to deal with.
There may, further, be cases where the effects of luminous radiative
repulsion become apparent, and also Crookes’ vacuum-effects described
as “radiant matter.” Nor is it quite certain that Laplace’s proofs of
the instantaneous propagation of gravity are final.
And in the future, as in the past, Tycho Brahe’s dictum must be
maintained, that all theory shall be preceded by accurate observations.
It is the pride of astronomers that their science stands above all
others in the accuracy of the facts observed, as well as in the rigid
logic of the mathematics used for interpreting these facts.
It is interesting to trace historically the invention of those
instruments of precision which have led to this result, and, without
entering on the details required in a practical handbook, to note the
guiding principles of construction in different ages.
It is very probable that the Chaldeans may have made spheres, like the
armillary sphere, for representing the poles of the heavens; and with
rings to show the ecliptic and zodiac, as well as the equinoctial and
solstitial colures; but we have no record. We only know that the tower
of Belus, on an eminence, was their observatory. We have, however,
distinct records of two such spheres used by the Chinese about 2500
B.C. Gnomons, or some kind of sundial, were used by the Egyptians and
others; and many of the ancient nations measured the obliquity of the
ecliptic by the shadows of a vertical column in summer and winter. The
natural horizon was the only instrument of precision used by those who
determined star positions by the directions of their risings and
settings; while in those days the clepsydra, or waterclock, was the
best instrument for comparing their times of rising and setting.
About 300 B.C. an observatory fitted with circular instruments for star
positions was set up at Alexandria, the then centre of civilisation. We
know almost nothing about the instruments used by Hipparchus in
preparing his star catalogues and his lunar and solar tables; but the
invention of the astrolabe is attributed to him.[1]
In more modern times Nuremberg became a centre of astronomical culture.
Waltherus, of that town, made really accurate observations of star
altitudes, and of the distances between stars; and in 1484 A.D. he used
a kind of clock. Tycho Brahe tried these, but discarded them as being
inaccurate.
Tycho Brahe (1546-1601 A.D.) made great improvements in armillary
spheres, quadrants, sextants, and large celestial globes. With these he
measured the positions of stars, or the distance of a comet from
several known stars. He has left us full descriptions of them,
illustrated by excellent engravings. Previous to his time such
instruments were made of wood. Tycho always used metal. He paid the
greatest attention to the stability of mounting, to the orientation of
his instruments, to the graduation of the arcs by the then new method
of transversals, and to the aperture sight used upon his pointer. There
were no telescopes in his day, and no pendulum clocks. He recognised
the fact that there must be instrumental errors. He made these as small
as was possible, measured their amount, and corrected his observations.
His table of refractions enabled him to abolish the error due to our
atmosphere so far as it could affect naked-eye observations. The
azimuth circle of Tycho’s largest quadrant had a diameter of nine feet,
and the quadrant a radius of six feet. He introduced the mural quadrant
for meridian observations.[2]
[Illustration: ANCIENT CHINESE INSTRUMENTS,
Including quadrant, celestial globe, and two armillae, in the
Observatory at Peking. Photographed in Peking by the author in 1875,
and stolen by the Germans when the Embassies were relieved by the
allies in 1900.]
The French Jesuits at Peking, in the seventeenth century, helped the
Chinese in their astronomy. In 1875 the writer saw and photographed, on
that part of the wall of Peking used by the Mandarins as an
observatory, the six instruments handsomely designed by Father
Verbiest, copied from the instruments of Tycho Brahe, and embellished
with Chinese dragons and emblems cast on the supports. He also saw
there two old instruments (which he was told were Arabic) of date 1279,
by Ko Show-King, astronomer to Koblai Khan, the grandson of Chenghis
Khan. One of these last is nearly identical with the armillae of Tycho;
and the other with his “armillae æquatoriæ maximæ,” with which he
observed the comet of 1585, besides fixed stars and planets.[3]
The discovery by Galileo of the isochronism of the pendulum, followed
by Huyghens’s adaptation of that principle to clocks, has been one of
the greatest aids to accurate observation. About the same time an
equally beneficial step was the employment of the telescope as a
pointer; not the Galilean with concave eye-piece, but with a magnifying
glass to examine the focal image, at which also a fixed mark could be
placed. Kepler was the first to suggest this. Gascoigne was the first
to use it. Huyghens used a metal strip of variable width in the focus,
as a micrometer to cover a planetary disc, and so to measure the width
covered by the planet. The Marquis Malvasia, in 1662, described the
network of fine silver threads at right angles, which he used in the
focus, much as we do now.
In the hands of such a skilful man as Tycho Brahe, the old open sights,
even without clocks, served their purpose sufficiently well to enable
Kepler to discover the true theory of the solar system. But telescopic
sights and clocks were required for proving some of Newton’s theories
of planetary perturbations. Picard’s observations at Paris from 1667
onwards seem to embody the first use of the telescope as a pointer. He
was also the first to introduce the use of Huyghens’s clocks for
observing the right ascension of stars. Olaus Romer was born at
Copenhagen in 1644. In 1675, by careful study of the times of eclipses
of Jupiter’s satellites, he discovered that light took time to traverse
space. Its velocity is 186,000 miles per second. In 1681 he took up his
duties as astronomer at Copenhagen, and built the first transit circle
on a window-sill of his house. The iron axis was five feet long and one
and a-half inches thick, and the telescope was fixed near one end with
a counterpoise. The telescope-tube was a double cone, to prevent
flexure. Three horizontal and three vertical wires were used in the
focus. These were illuminated by a speculum, near the object-glass,
reflecting the light from a lantern placed over the axis, the upper
part of the telescope-tube being partly cut away to admit the light. A
divided circle, with pointer and reading microscope, was provided for
reading the declination. He realised the superiority of a circle with
graduations over a much larger quadrant. The collimation error was
found by reversing the instrument and using a terrestrial mark, the
azimuth error by star observations. The time was expressed in fractions
of a second. He also constructed a telescope with equatoreal mounting,
to follow a star by one axial motion. In 1728 his instruments and
observation records were destroyed by fire.
Hevelius had introduced the vernier and tangent screw in his
measurement of arc graduations. His observatory and records were burnt
to the ground in 1679. Though an old man, he started afresh, and left
behind him a catalogue of 1,500 stars.
Flamsteed began his duties at Greenwich Observatory, as first
Astronomer Royal, in 1676, with very poor instruments. In 1683 he put
up a mural arc of 140°, and in 1689 a better one, seventy-nine inches
radius. He conducted his measurements with great skill, and introduced
new methods to attain accuracy, using certain stars for determining the
errors of his instruments; and he always reduced his observations to a
form in which they could be readily used. He introduced new methods for
determining the position of the equinox and the right ascension of a
fundamental star. He produced a catalogue of 2,935 stars. He supplied
Sir Isaac Newton with results of observation required in his
theoretical calculations. He died in 1719.
Halley succeeded Flamsteed to find that the whole place had been gutted
by the latter’s executors. In 1721 he got a transit instrument, and in
1726 a mural quadrant by Graham. His successor in 1742, Bradley,
replaced this by a fine brass quadrant, eight feet radius, by Bird; and
Bradley’s zenith sector was purchased for the observatory. An
instrument like this, specially designed for zenith stars, is capable
of greater rigidity than a more universal instrument; and there is no
trouble with refraction in the zenith. For these reasons Bradley had
set up this instrument at Kew, to attempt the proof of the earth’s
motion by observing the annual parallax of stars. He certainly found an
annual variation of zenith distance, but not at the times of year
required by the parallax. This led him to the discovery of the
“aberration” of light and of nutation. Bradley has been described as
the founder of the modern system of accurate observation. He died in
1762, leaving behind him thirteen folio volumes of valuable but
unreduced observations. Those relating to the stars were reduced by
Bessel and published in 1818, at Königsberg, in his well-known standard
work, _Fundamenta Astronomiae_. In it are results showing the laws of
refraction, with tables of its amount, the maximum value of aberration,
and other constants.
Bradley was succeeded by Bliss, and he by Maskelyne (1765), who carried
on excellent work, and laid the foundations of the Nautical Almanac
(1767). Just before his death he induced the Government to replace
Bird’s quadrant by a fine new mural _circle_, six feet in diameter, by
Troughton, the divisions being read off by microscopes fixed on piers
opposite to the divided circle. In this instrument the micrometer
screw, with a divided circle for turning it, was applied for bringing
the micrometer wire actually in line with a division on the circle—a
plan which is still always adopted.
Pond succeeded Maskelyne in 1811, and was the first to use this
instrument. From now onwards the places of stars were referred to the
pole, not to the zenith; the zero being obtained from measures on
circumpolar stars. Standard stars were used for giving the clock error.
In 1816 a new transit instrument, by Troughton, was added, and from
this date the Greenwich star places have maintained the very highest
accuracy.
George Biddell Airy, Seventh Astronomer Royal,[4] commenced his
Greenwich labours in 1835. His first and greatest reformation in the
work of the observatory was one he had already established at
Cambridge, and is now universally adopted. He held that an observation
is not completed until it has been reduced to a useful form; and in the
case of the sun, moon, and planets these results were, in every case,
compared with the tables, and the tabular error printed.
Airy was firmly impressed with the object for which Charles II. had
wisely founded the observatory in connection with navigation, and for
observations of the moon. Whenever a meridian transit of the moon could
be observed this was done. But, even so, there are periods in the month
when the moon is too near the sun for a transit to be well observed.
Also weather interferes with many meridian observations. To render the
lunar observations more continuous, Airy employed Troughton’s
successor, James Simms, in conjunction with the engineers, Ransome and
May, to construct an altazimuth with three-foot circles, and a
five-foot telescope, in 1847. The result was that the number of lunar
observations was immediately increased threefold, many of them being in
a part of the moon’s orbit which had previously been bare of
observations. From that date the Greenwich lunar observations have been
a model and a standard for the whole world.
Airy also undertook to superintend the reduction of all Greenwich lunar
observations from 1750 to 1830. The value of this laborious work, which
was completed in 1848, cannot be over-estimated.
The demands of astronomy, especially in regard to small minor planets,
required a transit instrument and mural circle with a more powerful
telescope. Airy combined the functions of both, and employed the same
constructors as before to make a _transit-circle_ with a telescope of
eleven and a-half feet focus and a circle of six-feet diameter, the
object-glass being eight inches in diameter.
Airy, like Bradley, was impressed with the advantage of employing stars
in the zenith for determining the fundamental constants of astronomy.
He devised a _reflex zenith tube_, in which the zenith point was
determined by reflection from a surface of mercury. The design was so
simple, and seemed so perfect, that great expectations were
entertained. But unaccountable variations comparable with those of the
transit circle appeared, and the instrument was put out of use until
1903, when the present Astronomer Royal noticed that the irregularities
could be allowed for, being due to that remarkable variation in the
position of the earth’s axis included in circles of about six yards
diameter at the north and south poles, discovered at the end of the
nineteenth century. The instrument is now being used for investigating
these variations; and in the year 1907 as many as 1,545 observations of
stars were made with the reflex zenith tube.
In connection with zenith telescopes it must be stated that Respighi,
at the Capitol Observatory at Rome, made use of a deep well with a
level mercury surface at the bottom and a telescope at the top pointing
downwards, which the writer saw in 1871. The reflection of the
micrometer wires and of a star very near the zenith (but not quite in
the zenith) can be observed together. His mercury trough was a circular
plane surface with a shallow edge to retain the mercury. The surface
quickly came to rest after disturbance by street traffic.
Sir W. M. H. Christie, Eighth Astronomer Royal, took up his duties in
that capacity in 1881. Besides a larger altazimuth that he erected in
1898, he has widened the field of operations at Greenwich by the
extensive use of photography and the establishment of large
equatoreals. From the point of view of instruments of precision, one of
the most important new features is the astrographic equatoreal, set up
in 1892 and used for the Greenwich section of the great astrographic
chart just completed. Photography has come to be of use, not only for
depicting the sun and moon, comets and nebulae, but also to obtain
accurate relative positions of neighbouring stars; to pick up objects
that are invisible in any telescope; and, most of all perhaps, in
fixing the positions of faint satellites. Thus Saturn’s distant
satellite, Phoebe, and the sixth and seventh satellites of Jupiter,
have been followed regularly in their courses at Greenwich ever since
their discovery with the thirty-inch reflector (erected in 1897); and
while doing so Mr. Melotte made, in 1908, the splendid discovery on
some of the photographic plates of an eighth satellite of Jupiter, at
an enormous distance from the planet. From observations in the early
part of 1908, over a limited arc of its orbit, before Jupiter
approached the sun, Mr. Cowell computed a retrograde orbit and
calculated the future positions of this satellite, which enabled Mr.
Melotte to find it again in the autumn—a great triumph both of
calculation and of photographic observation. This satellite has never
been seen, and has been photographed only at Greenwich, Heidelberg, and
the Lick Observatory.
Greenwich Observatory has been here selected for tracing the progress
of accurate measurement. But there is one instrument of great value,
the heliometer, which is not used at Greenwich. This serves the purpose
of a double image micrometer, and is made by dividing the object-glass
of a telescope along a diameter. Each half is mounted so as to slide a
distance of several inches each way on an arc whose centre is the
focus. The amount of the movement can be accurately read. Thus two
fields of view overlap, and the adjustment is made to bring an image of
one star over that of another star, and then to do the same by a
displacement in the opposite direction. The total movement of the
half-object glass is double the distance between the star images in the
focal plane. Such an instrument has long been established at Oxford,
and German astronomers have made great use of it. But in the hands of
Sir David Gill (late His Majesty’s Astronomer at the Cape of Good
Hope), and especially in his great researches on Solar and on Stellar
parallax, it has been recognised as an instrument of the very highest
accuracy, measuring the distance between stars correctly to less than a
tenth of a second of arc.
The superiority of the heliometer over all other devices (except
photography) for measuring small angles has been specially brought into
prominence by Sir David Gill’s researches on the distance of the
sun—_i.e.,_ the scale of the solar system. A measurement of the
distance of any planet fixes the scale, and, as Venus approaches the
earth most nearly of all the planets, it used to be supposed that a
Transit of Venus offered the best opportunity for such measurement,
especially as it was thought that, as Venus entered on the solar disc,
the sweep of light round the dark disc of Venus would enable a very
precise observation to be made. The Transit of Venus in 1874, in which
the present writer assisted, overthrew this delusion.
In 1877 Sir David Gill used Lord Crawford’s heliometer at the Island of
Ascension to measure the parallax of Mars in opposition, and found the
sun’s distance 93,080,000 miles. He considered that, while the
superiority of the heliometer had been proved, the results would be
still better with the points of light shown by minor planets rather
than with the disc of Mars.
In 1888-9, at the Cape, he observed the minor planets Iris, Victoria,
and Sappho, and secured the co-operation of four other heliometers. His
final result was 92,870,000 miles, the parallax being 8",802 (_Cape
Obs_., Vol. VI.).
So delicate were these measures that Gill detected a minute periodic
error of theory of twenty-seven days, owing to a periodically erroneous
position of the centre of gravity of the earth and moon to which the
position of the observer was referred. This led him to correct the mass
of the moon, and to fix its ratio to the earth’s mass = 0.012240.
Another method of getting the distance from the sun is to measure the
velocity of the earth’s orbital motion, giving the circumference
traversed in a year, and so the radius of the orbit. This has been done
by comparing observation and experiment. The aberration of light is an
angle 20” 48, giving the ratio of the earth’s velocity to the velocity
of light. The velocity of light is 186,000 miles a second; whence the
distance to the sun is 92,780,000 miles. There seems, however, to be
some uncertainty about the true value of the aberration, any
determination of which is subject to irregularities due to the
“seasonal errors.” The velocity of light was experimentally found, in
1862, by Fizeau and Foucault, each using an independent method. These
methods have been developed, and new values found, by Cornu,
Michaelson, Newcomb, and the present writer.
Quite lately Halm, at the Cape of Good Hope, measured spectroscopically
the velocity of the earth to and from a star by observations taken six
months apart. Thence he obtained an accurate value of the sun’s
distance.[5]
But the remarkably erratic minor planet, Eros, discovered by Witte in
1898, approaches the earth within 15,000,000 miles at rare intervals,
and, with the aid of photography, will certainly give us the best
result. A large number of observatories combined to observe the
opposition of 1900. Their results are not yet completely reduced, but
the best value deduced so far for the parallax[6] is 8".807 ±
0".0028.[7]
FOOTNOTES:
[1] In 1480 Martin Behaim, of Nuremberg, produced his _astrolabe_ for
measuring the latitude, by observation of the sun, at sea. It
consisted of a graduated metal circle, suspended by a ring which was
passed over the thumb, and hung vertically. A pointer was fixed to a
pin at the centre. This arm, called the _alhidada_, worked round the
graduated circle, and was pointed to the sun. The altitude of the sun
was thus determined, and, by help of solar tables, the latitude could
be found from observations made at apparent noon.
[2] See illustration on p. 76.
[3] See Dreyer’s article on these instruments in _Copernicus_, Vol. I.
They were stolen by the Germans after the relief of the Embassies, in
1900. The best description of these instruments is probably that
contained in an interesting volume, which may be seen in the library
of the R. A. S., entitled _Chinese Researches_, by Alexander Wyllie
(Shanghai, 1897).
[4] Sir George Airy was very jealous of this honourable title. He
rightly held that there is only one Astronomer Royal at a time, as
there is only one Mikado, one Dalai Lama. He said that His Majesty’s
Astronomer at the Cape of Good Hope, His Majesty’s Astronomer for
Scotland, and His Majesty’s Astronomer for Ireland are not called
Astronomers Royal.
[5] _Annals of the Cape Observatory_, vol. x., part 3.
[6] The parallax of the sun is the angle subtended by the earth’s
radius at the sun’s distance.
[7] A. R. Hinks, R.A.S.; Monthly Notices, June, 1909.
11. HISTORY OF THE TELESCOPE
Accounts of wonderful optical experiments by Roger Bacon (who died in
1292), and in the sixteenth century by Digges, Baptista Porta, and
Antonio de Dominis (Grant, _Hist. Ph. Ast_.), have led some to suppose
that they invented the telescope. The writer considers that it is more
likely that these notes refer to a kind of _camera obscura_, in which a
lens throws an inverted image of a landscape on the wall.
The first telescopes were made in Holland, the originator being either
Henry Lipperhey,[1] Zacharias Jansen, or James Metius, and the date
1608 or earlier.
In 1609 Galileo, being in Venice, heard of the invention, went home and
worked out the theory, and made a similar telescope. These telescopes
were all made with a convex object-glass and a concave eye-lens, and
this type is spoken of as the Galilean telescope. Its defects are that
it has no real focus where cross-wires can be placed, and that the
field of view is very small. Kepler suggested the convex eye-lens in
1611, and Scheiner claimed to have used one in 1617. But it was
Huyghens who really introduced them. In the seventeenth century
telescopes were made of great length, going up to 300 feet. Huyghens
also invented the compound eye-piece that bears his name, made of two
convex lenses to diminish spherical aberration.
But the defects of colour remained, although their cause was unknown
until Newton carried out his experiments on dispersion and the solar
spectrum. To overcome the spherical aberration James Gregory,[2] of
Aberdeen and Edinburgh, in 1663, in his _Optica Promota_, proposed a
reflecting speculum of parabolic form. But it was Newton, about 1666,
who first made a reflecting telescope; and he did it with the object of
avoiding colour dispersion.
Some time elapsed before reflectors were much used. Pound and Bradley
used one presented to the Royal Society by Hadley in 1723. Hawksbee,
Bradley, and Molyneaux made some. But James Short, of Edinburgh, made
many excellent Gregorian reflectors from 1732 till his death in 1768.
Newton’s trouble with refractors, chromatic aberration, remained
insurmountable until John Dollond (born 1706, died 1761), after many
experiments, found out how to make an achromatic lens out of two
lenses—one of crown glass, the other of flint glass—to destroy the
colour, in a way originally suggested by Euler. He soon acquired a
great reputation for his telescopes of moderate size; but there was a
difficulty in making flint-glass lenses of large size. The first actual
inventor and constructor of an achromatic telescope was Chester Moor
Hall, who was not in trade, and did not patent it. Towards the close of
the eighteenth century a Swiss named Guinand at last succeeded in
producing larger flint-glass discs free from striae. Frauenhofer, of
Munich, took him up in 1805, and soon produced, among others, Struve’s
Dorpat refractor of 9.9 inches diameter and 13.5 feet focal length, and
another, of 12 inches diameter and 18 feet focal length, for Lamont, of
Munich.
In the nineteenth century gigantic _reflectors_ have been made.
Lassel’s 2-foot reflector, made by himself, did much good work, and
discovered four new satellites. But Lord Rosse’s 6-foot reflector, 54
feet focal length, constructed in 1845, is still the largest ever made.
The imperfections of our atmosphere are against the use of such large
apertures, unless it be on high mountains. During the last half century
excellent specula have been made of silvered glass, and Dr. Common’s
5-foot speculum (removed, since his death, to Harvard) has done
excellent work. Then there are the 5-foot Yerkes reflector at Chicago,
and the 4-foot by Grubb at Melbourne.
Passing now from these large reflectors to refractors, further
improvements have been made in the manufacture of glass by Chance, of
Birmingham, Feil and Mantois, of Paris, and Schott, of Jena; while
specialists in grinding lenses, like Alvan Clark, of the U.S.A., and
others, have produced many large refractors.
Cooke, of York, made an object-glass, 25-inch diameter, for Newall, of
Gateshead, which has done splendid work at Cambridge. We have the
Washington 26-inch by Clark, the Vienna 27-inch by Grubb, the Nice
29½-inch by Gautier, the Pulkowa 30-inch by Clark. Then there was the
sensation of Clark’s 36-inch for the Lick Observatory in California,
and finally his _tour de force_, the Yerkes 40-inch refractor, for
Chicago.
At Greenwich there is the 28-inch photographic refractor, and the
Thompson equatoreal by Grubb, carrying both the 26-inch photographic
refractor and the 30-inch reflector. At the Cape of Good Hope we find
Mr. Frank McClean’s 24-inch refractor, with an object-glass prism for
spectroscopic work.
It would be out of place to describe here the practical adjuncts of a
modern equatoreal—the adjustments for pointing it, the clock for
driving it, the position-micrometer and various eye-pieces, the
photographic and spectroscopic attachments, the revolving domes,
observing seats, and rising floors and different forms of mounting, the
siderostats and coelostats, and other convenient adjuncts, besides the
registering chronograph and numerous facilities for aiding observation.
On each of these a chapter might be written; but the most important
part of the whole outfit is the man behind the telescope, and it is
with him that a history is more especially concerned.
SPECTROSCOPE.
Since the invention of the telescope no discovery has given so great an
impetus to astronomical physics as the spectroscope; and in giving us
information about the systems of stars and their proper motions it
rivals the telescope.
Frauenhofer, at the beginning of the nineteenth century, while applying
Dollond’s discovery to make large achromatic telescopes, studied the
dispersion of light by a prism. Admitting the light of the sun through
a narrow slit in a window-shutter, an inverted image of the slit can be
thrown, by a lens of suitable focal length, on the wall opposite. If a
wedge or prism of glass be interposed, the image is deflected to one
side; but, as Newton had shown, the images formed by the different
colours of which white light is composed are deflected to different
extents—the violet most, the red least. The number of colours forming
images is so numerous as to form a continuous spectrum on the wall with
all the colours—red, orange, yellow, green, blue, indigo, and violet.
But Frauenhofer found with a narrow slit, well focussed by the lens,
that some colours were missing in the white light of the sun, and these
were shown by dark lines across the spectrum. These are the Frauenhofer
lines, some of which he named by the letters of the alphabet. The D
line is a very marked one in the yellow. These dark lines in the solar
spectrum had already been observed by Wollaston.[3]
On examining artificial lights it was found that incandescent solids
and liquids (including the carbon glowing in a white gas flame) give
continuous spectra; gases, except under enormous pressure, give bright
lines. If sodium or common salt be thrown on the colourless flame of a
spirit lamp, it gives it a yellow colour, and its spectrum is a bright
yellow line agreeing in position with line D of the solar spectrum.
In 1832 Sir David Brewster found some of the solar black lines
increased in strength towards sunset, and attributed them to absorption
in the earth’s atmosphere. He suggested that the others were due to
absorption in the sun’s atmosphere. Thereupon Professor J. D. Forbes
pointed out that during a nearly total eclipse the lines ought to be
strengthened in the same way; as that part of the sun’s light, coming
from its edge, passes through a great distance in the sun’s atmosphere.
He tried this with the annular eclipse of 1836, with a negative result
which has never been accounted for, and which seemed to condemn
Brewster’s view.
In 1859 Kirchoff, on repeating Frauenhofer’s experiment, found that, if
a spirit lamp with salt in the flame were placed in the path of the
light, the black D line is intensified. He also found that, if he used
a limelight instead of the sunlight and passed it through the flame
with salt, the spectrum showed the D line black; or the vapour of
sodium absorbs the same light that it radiates. This proved to him the
existence of sodium in the sun’s atmosphere.[4] Iron, calcium, and
other elements were soon detected in the same way.
Extensive laboratory researches (still incomplete) have been carried
out to catalogue (according to their wave-length on the undulatory
theory of light) all the lines of each chemical element, under all
conditions of temperature and pressure. At the same time, all the lines
have been catalogued in the light of the sun and the brighter of the
stars.
Another method of obtaining spectra had long been known, by
transmission through, or reflection from, a grating of equidistant
lines ruled upon glass or metal. H. A. Rowland developed the art of
constructing these gratings, which requires great technical skill, and
for this astronomers owe him a debt of gratitude.
In 1842 Doppler[5] proved that the colour of a luminous body, like the
pitch or note of a sounding body, must be changed by velocity of
approach or recession. Everyone has noticed on a railway that, on
meeting a locomotive whistling, the note is lowered after the engine
has passed. The pitch of a sound or the colour of a light depends on
the number of waves striking the ear or eye in a second. This number is
increased by approach and lowered by recession.
Thus, by comparing the spectrum of a star alongside a spectrum of
hydrogen, we may see all the lines, and be sure that there is hydrogen
in the star; yet the lines in the star-spectrum may be all slightly
displaced to one side of the lines of the comparison spectrum. If
towards the violet end, it means mutual approach of the star and earth;
if to the red end, it means recession. The displacement of lines does
not tell us whether the motion is in the star, the earth, or both. The
displacement of the lines being measured, we can calculate the rate of
approach or recession in miles per second.
In 1868 Huggins[6] succeeded in thus measuring the velocities of stars
in the direction of the line of sight.
In 1873 Vogel[7] compared the spectra of the sun’s East (approaching)
limb and West (receding) limb, and the displacement of lines endorsed
the theory. This last observation was suggested by Zöllner.
FOOTNOTES:
[1] In the _Encyclopaedia Britannica_, article “Telescope,” and in
Grant’s _Physical Astronomy_, good reasons are given for awarding the
honour to Lipperhey.
[2] Will the indulgent reader excuse an anecdote which may encourage
some workers who may have found their mathematics defective through
want of use? James Gregory’s nephew David had a heap of MS. notes by
Newton. These descended to a Miss Gregory, of Edinburgh, who handed
them to the present writer, when an undergraduate at Cambridge, to
examine. After perusal, he lent them to his kindest of friends, J. C.
Adams (the discoverer of Neptune), for his opinion. Adams’s final
verdict was: “I fear they are of no value. It is pretty evident that,
when he wrote these notes, _Newton’s mathematics were a little
rusty_.”
[3] _R. S. Phil. Trans_.
[4] The experiment had been made before by one who did not understand
its meaning;. But Sir George G. Stokes had already given verbally the
true explanation of Frauenhofer lines.
[5] _Abh. d. Kön. Böhm. d. Wiss_., Bd. ii., 1841-42, p. 467. See also
Fizeau in the _Ann. de Chem. et de Phys_., 1870, p. 211.
[6] _R. S. Phil. Trans_., 1868.
[7] _Ast. Nach_., No. 1, 864.