CELESTIAL PHOTOGRAPHY (CONTINUED)—RECENT RESULTS.
Having in the previous chapter dealt with some of the pioneer work, we
come finally to consider some of the applications which in the last
years have occupied most attention.
With regard to the sun, we need scarcely say that Messrs. De La Rue and
Stewart have been enabled, by the photographic method, to give us data
of a most remarkable character, showing the periodicity of the changes
on the sun’s surface, and so establishing their correlation with
magnetic and other physical phenomena.
These photographic researches, following upon the eye observations of
Schwabe, Spörer, Carrington and others, have opened up to us a new field
of inquiry in connection with the meteorology of the globe; and it is
satisfactory to learn that photoheliographs are now daily at work at
Greenwich, Paris, Potsdam, and the Mauritius, and that shortly India
will be included in the list.
Quite recently, the importance of these permanent records of the solar
surface has been demonstrated by Dr. Janssen, the distinguished director
of the Physical Observatory at Meudon, in a very remarkable manner.
It seems a paradox that discoveries can be made depending on the
appearance of the sun’s surface by observations in which the eye applied
to the telescope is powerless; but this is the statement made by Dr.
Janssen himself, and there is little doubt that he has proved his point.
Before we come to the discovery itself let us say a little concerning
Dr. Janssen’s recent endeavours. Among the six large telescopes which
now form a part of the equipment of the new Physical Observatory
recently established by the French government at Meudon, in the grounds
of the princely Chateau there, is one to which Dr. Janssen has recently
almost exclusively confined his attention. It is a photoheliograph
giving images of the sun on an enormous scale—compared with which the
pictures obtained by the Kew photoheliograph are, so to speak, pigmies,
while the perfection of the image and the photographic processes
employed are so exquisite, that the finest mottling on the sun’s surface
cannot be overlooked by those even who are profoundly ignorant of the
interest which attaches to it.
This perfection of size and image have been obtained by Dr. Janssen by
combining all that is best in the principles utilised in one direction
by Mr. De La Rue, and in the other by Mr. Rutherfurd, to which we have
before referred. In the Kew photoheliograph, which has done such noble
work in its day that it will be regarded with the utmost veneration in
the future, we have first a small object-glass corrected after the
manner of photographic lenses, so as to make the so-called actinic and
the visual rays coincide, and then the image formed by this lens is
enlarged by a secondary magnifier constructed, though perhaps not too
accurately, so as to make the actinic and visual rays unite in a second
image on a prepared plate. Mr. Rutherfurd’s beautiful photographs of the
sun were obtained in a somewhat different manner. In his object-glass,
as we have seen, he discarded the visual rays altogether and brought
only the blue rays to a focus, but when enlargements were made, an
ordinary photographic lens—that is, one in which the blue and yellow
rays are made to coincide—was used.
Dr. Janssen uses a secondary magnifier, but with the assistance of M.
Pragmowski he has taken care that both it and the object-glass are
effective only for those rays which are most strongly photographic. Nor
is this all; he has not feared largely to increase the aperture and
focal length, so that the total length of the Kew instrument is less
than one-third of that in operation in Paris.
The largely-increased aperture which Dr. Janssen has given to his
instrument is a point of great importance. In the early days of solar
photography the aperture used was small, in order to prevent
over-exposure. It was soon found that this small aperture, as was to be
expected, produced poor images in consequence of the diffraction effects
brought about by it. It then became a question of increasing the
aperture while the exposure was reduced, and many forms of instantaneous
shutters have been suggested with this end in view. With these, if a
spring be used, the narrow slit which flashes across the beam to pay the
light out into the plate changes its velocity during its passage as the
tension of the spring changes. Of this again Dr. Janssen has not been
unmindful, and he has invented a contrivance in which the velocity is
constant during the whole length of run of the shutter.
By these various arrangements the plates have now been produced at
Meudon of fifteen inches diameter, showing details on the sun’s surface
subtending an angle of less than one second of arc.
So much for the _modus operandi_. Now for the branch of solar work which
has been advanced.
It is more than fifteen years ago since the question of the minute
structure of the solar photosphere was one of the questions of the day.
The so-called “mottling” had long been observed. The keen-eyed Dawes had
pointed out the thatch-like formation of the penumbra of spots, when one
day Mr. Nasmyth announced the discovery that the whole sun was covered
with objects resembling willow-leaves, most strangely and effectively
interlaced. We may sum up the work of many careful observers since that
time by stating that the mottling on the sun’s surface is due to
dome-like masses, and that the “thatch” of the penumbra is due to these
dome-like masses being drawn, either directly or in the manner of a
cyclone, towards the centre of the spot. In fact the “pores” in the
interval between the domes are so many small spots, while the faculæ are
the higher levels of the cloudy surface. The fact that faculæ are so
much better seen near the limb proves that the absorption of the solar
atmosphere rapidly changes between the levels reached by the upper
faculæ and the pores.
Thus much premised, we now come to Dr. Janssen’s discovery.
An attentive examination of his photographs shows that the surface of
the photosphere has not a constitution uniform in all its parts, _but
that it is divided into a series of figures more or less distant from
each other, and presenting a peculiar constitution_. These figures have
contours more or less rounded, often very rectilinear, and generally
resembling polygons. The dimensions of these figures are very variable;
they attain sometimes a minute and more in diameter.
While in the interior of the figures of which we speak the grains are
clear, distinctly terminated, although of very variable size, in the
boundary the grains are as if half effaced, stretched, stained; for the
most part, indeed, they have disappeared to make way for trains of
matter which have replaced the granulation. Everything indicates that in
these spaces, as in the penumbræ of spots, the photospheric matter is
submitted to violent movements which have confused the granular
elements.
We have already referred to the paradox that the sun’s appearance can
now be best studied without the eye applied to the telescope. This is
what Dr. Janssen says on that point.
“The photospheric network cannot be discovered by optical methods
applied directly to the sun. In fact, to ascertain it from the
plate, it is necessary to employ glasses which enabled us to embrace
a certain extent of the photographic image. Then if the magnifying
power is quite suitable, if the proof is quite pure, and especially
if it has received rigorously the proper exposure, it will be seen
that the granulation has not everywhere the same distinctness; that
the parts consisting of well-formed grains appear as currents which
circulate so as to circumscribe spaces where the phenomena present
the aspect we have described. But to establish this fact, it is
necessary to embrace a considerable portion of the solar disc, and
it is this which it is impossible to realise when we look at the sun
in a very powerful instrument, the field of which is, by the very
fact of its power, very small. In these conditions we may very
easily conclude that there exist portions where the granulation
ceases to be distinct or even visible; but it is impossible to
suppose that this fact is connected with a general system.”
But it is not alone with the uneclipsed sun that the new method enables
us to make discoveries. The extreme importance of photography in
reference to eclipse observations cannot be over estimated. Most of our
best observations of eclipses have been wrought by means of photography.
The time of an eclipse is an exciting time to astronomers; and it is
important that we should have some mechanical operation which should not
fail to record it.
[Illustration:
FIG. 212.—Copy of Photograph taken during the Eclipse of 1869.
]
The first eclipse photograph was taken in 1851. In 1860, chiefly owing
to the labours of Mr. De La Rue, our knowledge was enormously increased.
The Kew photoheliograph was the instrument used, and the series of
pictures obtained showed conclusively that the prominences belonged to
the sun. In 1868 the prominences were again photographed. In 1869 the
Americans attacked the corona, and their suggestion that the base of it
was truly solar has been confirmed by other photographs taken in 1870,
1871, and 1875. Although to the eye the phenomena changed from place to
place, to the camera it was everywhere the same with the same duration
of exposure.
* * * * *
It is not to be wondered at, then, that on the occasion of the last
transit of Venus, which may be regarded as a partial eclipse of the sun,
photography was suggested as a means of recording the phenomena.
Science is largely indebted to Dr. Janssen, Mr. De La Rue, and others
for bringing celestial photography to aid us in this branch of work
also. While on the one hand astronomers have to deal with precious
moments, to do very much in very little time, in circumstances of great
excitement; the photographer on the other goes on quietly preparing and
exposing his plates, and noting the time of the exposure, and thus can
make the whole time taken by the planet in its transit over the sun’s
disc one enormous base line. His micrometrical measures of the position
of the planet on the sun’s disc can be made after all is over. It was
suggested by Dr. Janssen that a circular plate of sufficient size to
contain sixty photographs of the limb of the sun, at the points at which
Venus entered and left it could be moved on step by step round its
centre, and so expose a fresh surface to the sun’s image focussed on it,
say every second. In this way the phenomena of the transit were actually
recorded at several stations.
* * * * *
[Illustration:
FIG. 213.—Part of Beer and Mädler’s Map of the Moon.
]
With reference to the moon, we have said enough to show that if we wish
to map her correctly, it is now no longer necessary to depend on
ordinary eye observations alone; it is perfectly clear that by means of
an image of the moon, taken by photography, we are able to fix many
points on the lunar surface. Still, although we can thus fix these and
use them as so many points of the first order, as one might say, in a
triangulation, there is much that photography cannot do; the work of the
eye observer would be essential in filling in the details and giving the
contour lines required to make a map of the moon.
The accompanying drawings on the same scale show that up to the present,
for minute work, the eye beats the camera.
[Illustration:
FIG. 214.—The same Region copied from a Photograph by De La Rue.
]
The light of the moon is so feeble in blue rays that a long exposure is
necessary for a large image, and during the exposure all the errors in
the rate of the clock are magnified.
We need not enlarge on the extreme importance of what Mr. Rutherfurd has
been doing in photographing star clusters and star groups. It is doubly
important to astronomy, and starts a new mode of using the equatorial
and the clock; in fact, it gives us a method by which observations may
be photographically made of the proper motion of stars, and even the
parallax of stars may be thus determined independently of any errors of
observers. Mr. Rutherfurd shows that the places of stars can be measured
by a micrometer on a plate in the same way as by ordinary observation;
hence photography can be made use of in the measurement of position and
distance of double stars.
As an instance of the extreme beauty of the photographs of stars
produced by a proper instrument, it may be stated that with the full
aperture of the 11¼-inch object-glass corrected only for the ordinary
rays, Mr. Rutherfurd found that he required an exposure of more than ten
seconds to get an image of the bright star Castor; but now, instead of
requiring ten seconds, he can get a better image in one. The reason of
this is, that, with the object-glass corrected only for the visual rays,
the chemical ones are spread over a certain small area instead of coming
to a point, and so, of course, the intensity is reduced; but when the
chemical rays all come to one point the intensity is greater, since the
image of the star is smaller and the action more intense.
Let us follow Mr. Rutherfurd a little in his actual work. First, a wet
plate is exposed for four minutes. This gives stars down to the tenth
magnitude. But there may be points on the plate which are not stars,
hence a second impression is taken on the same plate after it has been
slightly moved. All points now doubled are true stars. Now for measures
of arc. Another photograph is taken, and the driving clock is stopped;
the now moving stars down to the fourth magnitude are bright enough to
leave a continuous line, the length of this in a very accurately known
interval, say two minutes, enables the arc to be calculated.
Next comes the mapping. The negative is fixed on a horizontal divided
circle on glass illuminated from below. Above it is a system of two
rails, along which travels a carrier with two microscopes, magnifying
fifty diameters. By the one in the centre, with two cross wires in the
field of view, the photograph is observed; by the other, armed with a
wire micrometer, a divided scale on glass which is fixed alongside the
rail is read. Suppose we wish to measure the distance between two stars
on the plate. The plate is rotated, so that the line which joins them
coincides with that which is described by the optical axis of the
central microscope marked by the cross wires when the carrier runs along
the rails. This microscope is then brought successively over the two
stars, and the other microscope over the scale reads the nearest
division, while the fractions are measured by the micrometer. Hence,
then, the fixed scale, and not a micrometer screw, is depended upon for
the complete distance. In this way the distance between the stars on the
plate can be measured to the 1/500 part of a millimetre.
* * * * *
So far then we have shown how photography has been called in to the aid
of the astronomer, and how, by means of photography, pictures of the
different celestial bodies have been obtained of surpassing excellence.
Now, photography is also the handmaiden to the spectroscope in the same
way as it is the handmaiden to the telescope. Not only are we able to
determine and register the appearance of the moon and planets, but, day
by day, or hour by hour, we can photograph a large portion of the solar
spectrum; and not only so, but the spectrum of different portions of the
sun: nay, even the prominences have been photographed in the same
manner; while more recently still, Drs. Huggins and Draper have
succeeded in photographing the spectrum of some of the stars. We owe the
first spectrum of the sun, showing the various lines, to Becquerel and
Draper; the finest hitherto published we owe to Mr. Rutherfurd.
[Illustration:
FIG. 215.—Comparison between Kirchhoff’s Map and Rutherfurd’s
Photograph.
]
This magnificent spectrum extends from the green part of the spectrum
right into that part of the spectrum called the ultra-violet. Of course
it had to be put together from different pictures, because there is a
different length of exposure required for the different parts; the
exposure of any particular part of the spectrum must be varied according
to the amount of chemical intensity in that part. If the line G was
exposed, say for fifteen seconds, the spectrum near the line F would
require to be exposed for eight minutes, and at the line H, which is
further away from the luminous part of the spectrum than G, there the
exposure requisite would be two or three minutes.
[Illustration:
FIG. 216.—Arrangement for Photographically Determining the Coincidence
of Solar and Metallic Lines.
]
[Illustration:
FIG. 217.—Telespectroscope with Camera for obtaining Photographs of
the Solar Prominences.
]
In order to obtain a photograph of the average solar spectrum, the
camera replaces the observing telescope, and a heliostat is used, as in
the ordinary way. The beam, however, should be sent through an
opera-glass in order to condense it, and thereby to render the exposure
as short as possible.
Further, if an electric lamp be mounted as shown in Fig. 216,
observations, similar to those originally made by Kirchhoff, of the
coincidence on the various metallic lines with the Fraunhofer ones, can
be permanently recorded on the photographic plate. The lens between the
lamp and the heliostat is for the purpose of throwing an image of the
sun between the carbon poles. The lens between the lamp and spectroscope
then focuses both the poles and the image of the sun on to the slit. The
spectrum of the sun is first obtained by uncovering a small part of the
slit and allowing the image of the sun to fall on this uncovered
portion, the lamp not being in action. When this has been done the light
of the sun is shut off. The metal to be studied is placed in the lower
pole; the adjacent portion of the slit is uncovered, that at first used
being closed in the process. The current is then passed to render the
metal incandescent. After the proper exposure the plate is developed and
the spectra are seen side by side. Fig. 187 is a woodcut of a plate so
obtained.
If the spectrum of any special part of the sun, or the prominences, has
to be photographed, then either a siderostat must be employed, or a
camera is adjusted to the telespectroscope, as shown in Fig. 217.
For the stars, of course, much smaller dispersion must be used, but the
method is the same; and what has already been said by way of precaution
about the observation of stellar spectra applies equally to the attempt
to obtain spectrum photographs of these distant suns.
INDEX.
A.
Aberration (_see_ Chromatic Aberration, Spherical Aberration)
Absorption, general and selective, 403, 408;
spectroscope arranged for showing, 409
Adjustment of the transit instrument, 238
ADJUSTMENTS OF THE EQUATORIAL (Chap. XXI.), 328
Achromaticity of Huyghen’s eyepiece, 110
Achromatic lenses, 84, 86
Achromatism, 126
Airy’s transit circle, 284
Alexandrian Museum, astronomical observations, 19
Alt-azimuth, 287, 289
Altitudes, instrument used by Ptolemy for measuring, 35
Aluminium, line spectrum of, 406;
the sun, 417
Analyser for polarization of light, 443, 450
Anaximander, his theory of the form of the earth, 6;
invention of the gnomon ascribed to him, 16, 17;
meridian observations by, 25
Anchor escapement, 197
Angles of position, measurement of, 358-366, 372
Ångström, spectrum analysis, 402, 412;
wave-lengths, 406
Annealing of lenses and specula, 121
Archimedes, clocks used by, 176
Arcturus, heat of, 385
Argelander, magnitudes of stars, 382
Aries, its position in the zodiac, 34
Aristillus, his observations in the Alexandrian Museum, 19
_Armillæ Æquatoriæ_ of Tycho Brahe, 26, 41, 45;
his _Armillæ Zodiacales_, 28
Ascension, Right (_see_ Right Ascension)
Arctic circle, Euclid’s observations of stars in the, 10
Astrolabe, invented by Hipparchus, 25;
engraving of Tycho Brahe’s, 26, 41;
his ecliptic astrolabe, 28
Astronomical clock, 240 (_see_ Clock)
ASTRONOMICAL PHYSICS (Book VI.), 371
ASTRONOMY OF PRECISION, INSTRUMENTS USED IN (Chap. XIX.), 284-290
Astrophotometer, Zöllner’s, 379
Autolycus, first map of the stars by, 8, 9
Automatic spectroscope, 397
Auzout, invention of micrometer ascribed to, 219, 221
Axis of collimation, 218, 220
B.
Barium, in the sun, 419
Barlow, correction of aberration in lenses, 88;
“Barlow lenses,” 89, 229
Barometrical pressure, its effect on the pendulum, 193
Berthon’s dynameter, 116
Bessel’s transit instrument, 284
Binary stars, 351, 359, 360
Blair (Dr.), object-glasses, 88
Bloxam’s improved gravity escapement, 201
Bond (Prof.), spring governor, 320, 321;
celestial photography, 463
Bouguer’s photometer, 379
Brahe, Tycho (_see_ Tycho Brahe)
Brewster (Sir David), his list of Tycho Brahe’s instruments, 38;
spectrum analysis, 410
British Horological Institute, time signals, 280
Browning’s method of silvering glass specula, 137;
of mounting specula, 144;
automatic spectroscope, 397;
solar spectroscope, 428
Bunsen (Ernest de), on ancient astronomical observations, 6
Bunsen (Prof.) spectroscope, 396;
his burner, flame of, 407;
his work in spectrum analysis, 402, 412, 423
C.
Calcium, line spectra of, 406, 418
Cambridge Observatory (U.S.), equatorial at, 339;
star spectroscope, 430;
transit circle, 247, 248, 251
Camera, enlarging, for celestial photography, 458
Canada balsam, its power of refracting light, 447
Candles used to measure time, 176
Canopus, observations of, by Posidonius, 8
Cassegrain’s reflecting telescope, 103, 149, 169;
with Mr. Grubb’s mounting, 301
Casting lenses and specula, 121
Castor, photograph of, 478
Catalogues of stars (_see_ Stars)
Celestial globe, 23
CELESTIAL PHOTOGRAPHY (Chap. XXXI., XXXII.), 454
Chair, observing, for equatorial telescopes, 339
Chaldeans, their observations of the motions of the moon, 4;
early use of the gnomon, 16
Chance and Feil, manufacture of glass discs, 119, 305
CHEMISTRY OF THE STARS (Chap. XXVII.-XXX.), 386-453
Chinese, observations of conjunctions of planets, 4, 5;
early use of the gnomon, 16, 17
Chromatic aberration of object-glasses and eyepieces, 87, 109, 123
CHRONOGRAPH, THE (Chap. XVII.), 253-270
“Chronographic method” of transit observation, 259
Chronograph at Greenwich Observatory, 260-264
CHRONOMETER, THE (Chap. XIII.), rise and progress of time-keeping,
206-210;
compensating balance, 207;
detached lever escapement, 208;
chronometer escapement fusee, 209
Chronometers used for determining “local time,” 281
Chronophers, for distributing “Greenwich time,” 275, 276
Cincinnati Observatory, 338
Circle, the; its first application as an astronomical instrument, 6, 7,
8, 10;
division into degrees, 8, 17, 21
Circles, great, defined by Euclid, 12
CIRCLE READING (Chap. XIV.), 211-217;
Digges’ diagonal scale, 213;
the vernier, 214
CIRCLE, TRANSIT (_see_ Transit Circle)
Circle, meridian, at Cambridge (U.S.), 248;
mural, 241, 242
Circumpolar stars, 239
Clarke (Alvan), improvement in telescope lenses, 305;
great equatorial at Washington, 309, 319
Clement, inventor of the anchor escapement, 197
Clepsydras, 36
CLOCK, THE (Chap. XIII.), 175-205;
ancient escapement, 177;
crown wheel, 178;
clock train, 180;
winding arrangements, 181;
pendulum, 183;
cycloidal pendulum, 185;
compensating pendulums, 187;
Graham’s, Harrison’s, and Greenwich pendulums, 188;
clock at Royal Observatory, Greenwich, 194;
escapements, 196;
anchor escapement, 197;
Graham’s dead-beat, 199;
Mudge’s gravity escapement, 200;
escapement of clock at Greenwich, 203;
arrangements at Edinburgh Observatory, 269;
astronomical, 240, 244, 245, 346;
sidereal, 254, 256, 266;
solar, 254;
standard, at Greenwich, 194, 203, 204, 271, 274
Clock, driving, for large telescopes, 318
Clocks driven and controlled by electricity, 272
Clock stars, 267
Clock tower at Westminster, 277
Coggia’s comet, its light polarized, 450
Collimation and collimation-error in the transit instrument and
equatorial, 238, 247, 328
Colour, amount produced by a lens, 81, 84, 86;
spectrum analysis, 407, 408, 414, 416;
of stars, 165, 351, 433;
of waves of light, 420;
refrangibility of, 387
Comet of 1677, discovered by Tycho Brahe, 47
Comet, measurement of the angle of position of its axis, 359
Comparison prism of the spectroscope, 423
Compensating balance, 207
Compensating pendulums, 187-193
Composite mounting of large telescopes, 310
Concave lenses (_see_ Lenses)
Concave mirrors (_see_ Mirrors)
Conjugate images, 64
Conjunctions of planets, first observations, 4
Constellations, first observations, 5, 9;
Orion and its neighbourhood, 156
Convex lenses (_see_ Lenses)
Convex mirrors (_see_ Mirrors)
Cooke, adjustment of object-glasses, 141;
improvement in telescope lenses, 305;
equatorial refractor, 300;
driving clock for large telescopes, 321;
illuminating lamp for equatorial telescopes, 326
Copernicus, parallactic rules of, 41
Copernicus (lunar crater), 354
Cross wires for circle reading, 212, 216, 218;
in transit eyepiece, 234, 257
Crown-glass prisms, 83, 84;
lenses, 86, 88
Crystals of Iceland spar, double refraction by (_see_ Iceland Spar)
Culmination of stars, first observations of, 5
Cycloidal pendulum, 185
D.
Dawes, solar eyepiece, 114, 115, 349;
photometry, 378
Day, solar and sidereal, 253, 254, 256
Day eyepiece, 113
Days, first reckoning of, 19;
measurement of, 176
Dead-beat escapement, 198
Deal time-ball, 275, 279
Declination, 24, 234, 241, 243, 251;
measured by Tycho Brahe, 45
Declination axis of the equatorial, 299, 308, 327, 328
Defining power of the modern telescope, 160, 164;
stars in Orion a test of, 165
Degrees, division of the circle into, 8, 17, 21
De La Rue (Warren, F.R.S.), his reflecting telescope, 108;
improvements in polishing specula, 134;
celestial photography, 454, 459, 460, 464, 465, 475
Denderah, the zodiac of, 7
Dent (E. & Co.), clock at Royal Observatory, Greenwich, 194, 203, 204,
271, 274
Detached lever escapement, 208
Deviation of light, 79, 82
Deviation error in the transit instrument, 240, 248
Dials of ancient clocks, 257
Diagonal scale, Digges’, 213
Differential observations made with the equatorial, 367
Digges’ diagonal scale, 213
Diogenes Laertes, on the invention of the gnomon, 16
Dioptrics, Kepler’s treatise on, 386
Direct vision spectroscope, 431
Dispersion of light by prism, 79, 80, 82
Dividing power of telescopes, 165
Dollond, experiments with lenses, 85;
correction of chromatic aberration, 89;
on manufacture of flint-glass discs, 118;
pancratic eyepiece, 113
Dome form of observatory, 338, 339
Double stars, 351, 359;
measurement of, 360
Double-image micrometer, 225, 229
Double refraction by crystals of Iceland spar (_see_ Iceland Spar)
Driving clock, for large telescopes, 318, 346
Drum form of observatory, 338
Dundee time signal, 278
E.
Earth, The, its position in Ptolemy’s system, 3;
early theories of its form, 6;
circumference measured by Posidonius, 8;
Euclid’s theory of its position, 12;
inclination of its axis, 14, 17;
size measured by Eratosthenes, 19;
position in Tycho Brahe’s system, 46
Eclipses, first observations of, 4;
eclipses of Jupiter’s moons;
eclipses, solar, photograph of, 474
Ecliptic, plane of the, 13, 14;
discovery of its inclination, 17;
inclination measured by Eratosthenes, 19
Ecliptic astrolabe of Tycho Brahe, 28
Edinburgh Observatory, clock arrangements at, 269;
standard clock, 272;
time signals, 278
Egyptians, their record of eclipses, 4;
zodiac of Denderah, 7
Eichens, his equatorial telescope at Paris, 314, 315;
siderostat constructed by him, 344
Electricity, its application to the chronograph, 265;
to driving and controlling clocks, 272
Electric lamp, 404;
arranged for spectrum analysis, 405
Emery used in grinding lenses and specula, 127
English mounting of large telescopes, 310
Equation of time, 254
EQUATORIAL, THE (Book V.), 293-368 (_see_ Telescopes)
EQUATORIAL OBSERVATORY, THE (Chap. XXII.), 337-342 (_see_
Observatories)
EQUATORIAL, THE; its ordinary work, (Chap. XXIV.), 349-368
Equinoctial circle, observations of, by Euclid, 11
Equinoxes, first observations of, 15, 16, 17, 22;
precession of the, 33
Eratosthenes, observations of, 17;
his measurement of the earth, and inclination of the ecliptic, 19;
meridian circle invented by, 20
Erecting eyepiece, 113
Errors, collimation and deviation, in the transit instrument, 238, 240,
247, 328
Errors; personal equation, 259;
adjustments of the equatorial, 329
Ertel, vertical circle designed by, 290
Escapements of clocks, 196-205;
ancient, 177;
anchor, 197;
Graham’s, 199;
Mudge’s, 200;
Greenwich clock, 203;
detached lever, 208;
chronometer escapement, 209
Ethereal vibrations, 373, 401, 410, 420, 449, 450
Euclid, his observations of the stars, 8, 9, 10;
of great circles, horizon, meridian and tropics, 11, 12;
theory of the earth’s position, 12;
pole star, 14
Extra-meridional observations, first employment of, 23, 25
“Eye and ear” method in transit observations, 259
Eyeball, section of the, 66
Eyepieces, Huyghen’s, 110;
Ramsden’s, Dollond’s, 112;
erecting or “day eyepiece,” 112;
Dawes’s solar eyepiece, 114;
magnifying power of, 116
Eyepiece of Greenwich transit circle, 246;
of transit instrument, 257
F.
Faye, M., celestial photography, 456
Feil and Chance, manufacture of flint glass discs, 119, 305
Fixed stars (_see_ Stars)
Flame of salts in a Bunsen’s burner, 407
Flint-glass prisms, 83, 84;
lenses, 86, 170
Flint-glass, improvements in the manufacture of discs of, 118, 119, 305
Focal length of telescopes, 82, 458;
of lenses, 62, 63;
of convex mirrors, 94
Foucault; his reflecting telescope, 108;
improvement of specula, 117;
mode of polishing specula, 134, 136;
mounting of his telescope, 311;
governor of driving clock for large telescopes, 323;
siderostat, 343;
spectrum analysis, 410;
heliostat, 424
Fraunhofer; manufacture of flint-glass discs, 118;
large telescopes, 303;
lines in the solar spectrum, 392;
spectrum analysis, 402, 410, 422, 425, 432, 438
Frederick II. of Denmark, his patronage of Tycho Brahe, 38
Fusee for chronometers, 209
G.
Galileo; his telescopes, 73, 78;
their magnifying power, 77;
the pendulum, 183, 184
Gascoigne, eyepieces and circle reading, 212;
cross wires for “telescopic sight,” 219
Gateshead, Mr. Newall’s refractor, 302
Geissler’s tubes, 413
German mounting of large telescopes, 299
Gizeh, great pyramid of, an astronomical instrument, 6
Glasgow, electric time-gun, 278
Glass, injurious effects of the duty on, 305
Glass specula, methods of silvering, 137
Globe, celestial, 23;
terrestrial, 23
Gnomon; its invention and early use, 16;
improvements in, 18, 175
Graham; dead-beat escapement, 192, 199;
mercurial pendulum, 188
Gravity escapement, 200, 202
Greeks, their early use of the gnomon, 16
Greenwich, Royal Observatory; perspective view and plan of transit
circle, 243, 245, 251;
transit room, 251, 257;
meridian of, 252;
chronograph, 260-264;
computing room, 267;
standard sidereal clock, 267;
mean solar time clock, 268;
standard clock, 274;
pendulum, 188;
reflex zenith tube, 286;
alt-azimuth, 290;
equatorial, 310;
thermopile, 384;
photoheliograph, 469
“GREENWICH TIME” AND THE USE MADE OF IT (Chap. XVIII.), 271-283
Gregorian telescope, 149
Gridiron pendulum, 188, 189, 192
Grinding of lenses and specula, 127
Grubb; production and polishing of metallic specula, 121, 134;
adjustment of object-glasses, 141;
Cassegrainian and Newtonian reflectors, 102, 108, 301, 303;
great Melbourne equatorial telescope, 108, 314, 315, 317, 324, 327;
mode of mounting its speculum, 145-149;
automatic spectroscope, 397;
solar spectroscope, 428
Guinand, manufacture of flint-glass discs, 118
Guns fired as time-signals, 278
H.
Haliburton, on ancient astronomical observations, 6
Hall; experiments with lenses, 85;
manufacture of flint-glass discs, 118
Harcourt, Vernon, experiments with phosphatic glass, 123
Harrison’s gridiron pendulum, 188
HEAT OF STARS, DETERMINATION OF (Chap. XXVI.), 377-385
Heliometer, 224
Heliostat, 423, 458
Henry (Prof.), radiation of heat from sun-spots, 385
Herschel (Sir John), lenses corrected for aberration, 88;
table of reflective powers, 169;
star magnitudes, 381
Herschel, Sir William, his reflecting telescopes, 103, 108;
his mode of polishing specula, 129;
great telescope at Slough, 169, 294
Herschel-Browning direct-vision prism, 400
Hipparchus, trigonometrical tables constructed by, 17;
discoveries of, 25-35;
his measurement of space, 213
Hittorf, spectrum analysis, 413
Holmes (N. J.), his proposal of the electric time-gun, 278
Hooke, improvement in clock escapements, 196;
micrometer, 221, 222;
zenith sector invented by, 285;
siderostat suggested by, 343
Horizon, the first astronomical instrument, 4, 7, 8;
defined by Euclid, 12
Horological Institute, time-signals, 280
Hours, first reckoning of, 19;
measurement of, 176
Hour circle of the equatorial telescope, 328, 335
Huen, island of, granted to Tycho Brahe, 38
Huggins (Dr.), telespectroscope, 429, 432
Huyghens; telescopes used by, 81;
eyepiece, 110, 116, 212;
application of the pendulum to clocks, 183;
his measurements of space, 219, 223, 343;
polarized light, 442
Hydrogen in the sun, 435
I.
Iceland spar crystals; double refraction by, 226, 228;
polarization of light, 442, 445, 447, 449, 450
Illuminating power of the telescope, 158, 166, 168, 169;
stars in Orion, a test of, 164
Images, double, seen through Iceland spar, 227
Inclination of the earth’s axis, 14, 17
Inclination of the ecliptic, 17;
measured by Eratosthenes, 19
Index error, adjustments of the equatorial, 330
Iron, line spectrum of, 406, 418
Irrationality of the spectrum, 87
J.
Janssen (Dr.), solar photography, 471;
discoveries in solar physics, 472
Jupiter, in Ptolemy’s system, 3;
in Tycho Brahe’s, 46;
as a telescopic object, 351;
photographs of, 465, 466
Jupiter’s moons, observation of their eclipses to determine “local
time,” 282
K.
Kepler’s treatise on dioptrics, 386
Kew Observatory, photographs of the sun and sun-spots, 460, 465, 470,
475
Kirchhoff; spectroscope, 396;
spectrum analysis, 402, 403, 412, 422, 428
Kitchener (Dr.), improved eyepiece, 113;
stars in Orion, 164
Knobel’s photometer, 378
Knott, star magnitudes, 381
L.
Lamp for equatorial telescope, 325
Lamp, electric (_see_ Electric Lamp)
Lassell; his Newtonian telescope, 108, 311;
production, polishing, and mounting metallic specula, 121, 132, 144
Latitude; observations of Posidonius, 8;
parallels of, 23
Lattice-work for tubes of telescopes, 172
Lenses; action of, 55, 58, 85;
concave and convex, 61, 71, 75;
amount of colour produced by, 81;
achromatic, 84;
Hall and Dollond’s experiments, 85;
correction for colour, 87;
correction for aberration in eyepieces, 109, 116;
production of, 117
Lens, crystalline, of the eye, 67
Lewis (Sir G. C.), his “Astronomy of the Ancients,” 9
Liebig, improvement in specula, 117
Light; refraction, 55-72;
deviation and dispersion, 79, 80, 82, 83;
decomposition and recomposition, 83;
reflection, 90-99;
action of a reflecting surface, 91;
angles of incidence and reflection, 92;
concave and convex mirrors, 94-98;
velocity of, 159;
loss due to reflection, 168;
effective, in reflectors, 169;
vibration of particles, 373, 401;
polarization, 441-453
LIGHT OF STARS, DETERMINATION OF (Chap. XXVI.), 377-385
Lindsay (Lord), siderostat at his observatory, 347
Local time, 281
Longitude, meridians of, 23;
as determined by Hipparchus and Tycho Brahe, 44;
determined by clock and transit instrument, 280;
expressed in degrees and time, 280
M.
Magnesium vapour; colour of, 416;
in the sun, 435
Magnifying power of large telescopes, 154, 155;
stars in Orion, a test of, 163
Magnitude of stars, 377
Malus, discovery of polarization by reflection, 442, 448
Malvasia (Marquis), his micrometer, 219, 221
Manlius, gnomon erected by him at Rome, 18
Maps of the stars (_see_ Stars)
Mars, in Ptolemy’s system, 3;
in Tycho Brahe’s, 46;
as a telescopic object, 350
Martin’s method of silvering glass specula, 138
Mauritius, photoheliograph at, 469
Mean time, 254
Mean solar time clock at Greenwich, 268
Melbourne Observatory, great reflecting telescope, 312, 313, 337;
composition and production of specula, 120, 121, 129;
view of optical part, 143;
mode of mounting speculum, 144-149;
photographs of the moon, 459
Mercurial pendulum, 187, 188, 192
Mercury, in Ptolemy’s system, 3;
in Tycho Brahe’s, 46;
as a telescopic object, 350
Meridian, defined by Euclid, 12
Meridional observations, first employment of, 20
Meridian of Greenwich, 252
Meridian circle, the first, 20;
at Cambridge (U.S.), 248
Meridians of longitude (_see_ Longitude)
MERIDIONAL OBSERVATIONS, MODERN (Book IV.), 233-290
Merz (M.), manufacture of flint-glass discs, 119;
cost of large object-glasses, 172;
large telescopes, 303
Metallic specula, 120, 171
Meton, meridian observations by, 25
Meudon Observatory, solar photography at, 470
MICROMETER, THE (Chap. XV.), 218-232;
wire micrometer, 221, 352;
heliometer, 224;
double image, 229;
position, 353;
measurements made by, 355, 359-366, 368
Microscopes, for reading transit circles, 247;
for Newall’s telescope, 307
Middlesborough, time signal, 278
Milky Way, observations of Euclid, 11
Miller, spectrum analysis, 410
Mirrors, concave and convex, 94-98
Mirrors for reflecting telescopes (_see_ Specula)
MODERN MERIDIONAL OBSERVATIONS (Book IV.), 233-290
Molecular vibration, 373, 401, 410, 429, 449, 450
Months, first observations of, 5
Moon, The, in Ptolemy’s system, 3;
motions observed by the Chaldeans, 4;
parallax observed by Ptolemy, 35;
used by Hipparchus to determine longitude, 44;
as a telescopic object, 350;
the lunar crater, Copernicus, 354;
measurement of shadow thrown by a lunar hill, 355;
photographs and stereographs, 459, 464, 465, 466;
part of Beer and Mädler’s map, 476;
of De La Rue’s photograph, 477
MOUNTING OF LARGE TELESCOPES (Chap. XX.), 293-327
Mounting of specula for reflecting telescopes, 144, 149, 169
Mudge, grinding and polishing specula, 129;
gravity escapement, 200
Mural circle, 241, 242
Mural quadrant, Tycho Brahe’s, 233, 235
Multiple stars, 351
N.
Nebulæ, 351
Nebula of Orion, 157, 158
Neptune, as a telescopic object, 351
Newall’s equatorial refractor, 302;
with spectroscope, 427;
flint-glass discs for, 119;
production of discs for object-glass, 128;
photographs of the moon, 459
Newcastle, time signals, 278
Newton (Sir Isaac), on refracting telescopes, 82;
his reflecting telescope, 101, 102;
use of pitch in polishing specula, 128;
refrangibility of light, 387;
polarized light, 442
Newtonian reflector, 149;
view of optical part, 143;
effective light, 169;
Grubb’s form, 303;
Browning’s form, 304;
mounting of, 310
Nicols’ prism, 115;
measurement of the light of stars, 380;
polarization of light, 443, 447, 448, 449, 450
North pole, diagram illustrating how it is found, 249, 251
O.
Object-glasses, production of, 118, 119;
correction of colour, 88;
correction for spherical aberration, 126;
mode of polishing, 128;
mode of centring, 140;
illustrations of defective adjustment, 141;
adjustment of, 163;
its perfection in modern telescopes, 166, 305;
cost of production, 172;
divided, for duplication of image, 225
Object-glass prism, 426
Observatories [_see_ Alexandrian Museum, Cambridge (U.S.), Cincinnati,
Edinburgh, Greenwich, Huen (Tycho Brahe’s), Kew, Lord Lindsay’s,
Mauritius, Melbourne, Meudon, Paris, Potsdam, Vienna, Washington]
Observing chair for equatorial telescopes, 339
Optical action of the eye, 67;
long and short sight, 69, 71
Optical qualities of telescopes, permanence of, 170
Optic axis in crystals of Iceland spar, 228
“Optick tube,” telescope so first called, 55, 139-151
Orion, first observations of, 5;
Orion and the neighbouring constellations, 156;
nebula of, 157, 158;
stars in, a test for power of telescopes, 164-166;
facilities for observing, 164
P.
Parallactic rules, 51;
used by Ptolemy, 35;
by Tycho Brahe, 38, 41
Parallax of the moon, observed by Ptolemy, 35
Paris Observatory, reflecting equatorial telescope, 314, 315, 337;
siderostat, 344;
photoheliograph, 469
Pendulum, 183, 185, 187, 188
Personal equation, 259
Phosphatic glass for lenses, 123
PHOTOGRAPHY, CELESTIAL (Chap. XXXI., XXXII.), 454-483
Photography, stellar, 172
Photoheliograph, for photographs of the sun, 460, 470;
for transit of Venus (1874), 461
Photometry, 373, 377
PHYSICS, ASTRONOMICAL (Book VI.), 371
PHYSICAL INQUIRY, GENERAL FIELD OF (Chap. XXV.), 371-376
Picard, transit circle, 284
Pisces, its position in the zodiac, 34
Pitch employed in polishing lenses and specula, 128, 132
Plane of the ecliptic, 13, 14
Planets, in Ptolemy’s system, 3;
first observations of conjunction, 4, 5;
motions observed by Autolycus, 9;
in Tycho Brahe’s system, 46;
Saturn seen with object-glasses of 3¾ and 26 inches, 160, 161;
as telescopic objects, 350;
photographs of, 465
Pleiades, the first observations of, 5
Plücker, spectrum analysis, 413
Pogson, star magnitudes, 381, 382
Pointers of pre-telescopic instruments, 35, 49, 214, 216
Polar axis of the equatorial, 299, 302, 308, 311, 312, 324, 328, 329,
346
Polariscope, 441-453
Polarization of light, 441-453
Pole, North, 238;
diagram illustrating how it is found, 249
Pole star, first observations of, 6;
observations of Euclid, 10, 14;
its position, 238
Polishing lenses and specula, 128, 171;
Lord Rosse’s polishing machine, 131;
Mr. Lassell’s, 132
Posidonius, measurement of the earth’s circumference, 8
Position circle, 353
Position micrometer, 353, 358
Post Office Telegraphs, for distribution of Greenwich time, 275
Potsdam, photoheliograph at, 469
Precession of the equinoxes, 33
Prime-vertical, 285
Prime-vertical instrument, 287
Primum mobile of Ptolemy, 3
Prisms, action of, 55;
crown and flint-glass, 83, 84;
water, 85;
doubly refracting, for the micrometer, 226;
direct vision, 400;
in the spectroscope, 393-400;
object-glass prism, 426
Ptolemy, the Heavens according to, 3;
trigonometrical tables, 17;
sun’s altitude, 21;
his discoveries, 35;
parallax of the moon, 35;
his measurement of time, 36;
parallactic rules, 38, 51
Purbach, observation of altitudes by, 36
Pyramids, the first constructed astronomical instruments, 5, 6
Q.
Quadrants used by Tycho Brahe, 38;
his _quadrans maximus_, 48
Quadrant, mural, 233, 235
Quartz crystals for polarizing light, 450, 452
R.
Radiation of stars, visual, 383;
thermal, 385
Radiation, general and selective, 403, 408
Ramsden’s eyepiece, 112, 212
Reading microscopes, for Greenwich and Cambridge (U.S.) transit
circles, 247;
for Newall’s telescope, 307
Red stars (_see_ Colour of Stars)
Reflection of light (_see_ Light)
Reflecting telescopes (_see_ Telescope)
Reflective powers, Sir John Herschel’s table of, 168
Reflector, diagonal, for solar observations, 114
Reflecting and refracting telescopes compared, 170
Reflex zenith-tube at Greenwich, 286
Refracting telescopes (_see_ Telescopes)
Refracting and reflecting telescopes compared, 170
Refraction of light (_see_ Light)
Refraction, double, by crystals of Iceland spar (_see_ Iceland Spar)
Refrangibility of colours, 387;
of light, 420
Regiomontanus, altitudes measured by, 36
Regulation of clocks by electricity, 272
Rising of stars (_see_ Stars)
Right ascension, 24, 234, 241, 249, 257;
measured by Hipparchus, 44;
by Tycho Brahe, 45
Ring micrometer, 368
Robinson (Dr.),
specula of Melbourne telescope, 129;
apertures of object-glasses, 168
Rockets fired as time signals, 281
Römer, wires in a transit eyepiece, 220;
transit circle and transit instrument, 284
Rosse (Lord), his reflecting telescope, 108, 294, 311, 312;
composition of reflector, 120;
production of metallic specula, 121, 131;
nebula of Orion as seen by his reflector, 157, 158;
illuminating power of his telescope, 159;
effective light, 169;
thermopile observations, 384
Royal Observatory, Greenwich (_see_ Greenwich)
Rudolph II. (Emperor), his patronage of Tycho Brahe, 42
Rumford’s photometer, 377
Rutherfurd, his work in celestial photography, 455, 464, 466, 471, 477,
480
S.
Salts, flame of, in a Bunsen’s burner, 407
Sand clocks and sand glasses, 176
Saturn, in Ptolemy’s system, 3;
in Tycho Brahe’s, 46;
as seen with a 3¾ inch and 26 inch object-glass, 160, 161;
as a telescopic object, 351;
mode of measuring its rings, 357;
photographs of, 465, 466
Savart’s analyser for polarization of light, 452
Scarphie, employed by Eratosthenes, 19
Scheiner’s telescope, 78
Seasons, The, 15, 16
Secchi (Father), direct-vision star spectroscope, 431;
stellar spectra, 433
Setting of stars (_see_ Stars)
Sextants used by Tycho Brahe, 38, 50
Sidereal clock, 254, 266 (_see_ Clock)
Sidereal day, 256
Sidereal time, 240, 254, 324
SIDEROSTAT, THE (Chap. XXIII.), 343-348, 461;
at Lord Lindsay’s Observatory, 347
Signals for distributing “Greenwich time,” 278
Signals, time, 281, 283
Signs of the zodiac (_see_ Zodiac)
Silver-on-glass reflector at the Paris Observatory, 316
Silvering glass specula, modes of, 137;
silvered glass reflectors, 171
Simms, his introduction of the collimator in the spectroscope, 393, 425
Sirius, first observations of, 5;
spectrum of, 432
Slough, Sir Wm. Herschel’s telescope at, 294
Smyth (Admiral), stars in Orion, 165;
colours of stars, 351;
star magnitudes, 381
Smyth (Prof. Piazzi), on the pyramids as astronomical instruments, 6;
position of the vernal equinox, 34;
clock arrangements at Edinburgh Observatory, 269
Sodium, discovery of its presence in the sun, 412
Solar photography, 459, 465
Solar spectroscope, 435;
Browning’s and Grubb’s forms, 428
Solar spectrum, 390, 391, 392, 423, 433, 436, 438, 439;
photographs of, 479, 480
Solar time, 253, 255
Solstices, first observations of the, 15, 16, 17, 22
Southing of stars, 234
SPACE MEASURERS (Book III.), 135-232;
circle reading, 211;
Digges’ diagonal scale, 213;
the vernier, 214;
micrometers, 218
Space-penetrating power of the telescope, 154;
stars in Orion, a test of, 165
Spectroscope, construction of the, 393-400;
automatic, 397;
arranged for showing absorption, 409;
attached to Newall’s refractor, 427;
solar, Browning’s and Grubb’s forms, 428
Spectrum produced by prisms, irrationality of the, 86, 87
Spectrum, solar, 390, 391, 392
Spectrum analysis, principles of, 401-421
Specula, production of, 117, 120;
casting, annealing, 121;
curvature, 122;
grinding, 127;
polishing, 128;
silvering, 137;
mounting, 142, 169, 172;
effective light, 169;
repolishing, 171;
cost as compared with object-glasses, 172
Spherical aberration, 87;
diagram illustrating, 104, 105;
its correction in eyepieces, 109, 111;
of specula, 123, 124
Sprengel pump, 413
Spring governor of driving-clock for large telescopes, 319, 320
“Spurious disc” of fixed stars, 163
Standard clock at Edinburgh Observatory, 272
Standard sidereal clock of Greenwich Observatory, 267
Standard solar time clock of Greenwich Observatory, 267
STARS, CHEMISTRY OF THE (Chap. XXVII.-XXX.), 386-453
STARS, LIGHT AND HEAT OF (Chap. XXVI.), 377;
variable, 377-385
Stars, first observations of the, 4, 5, 6, 7;
first maps of, 8;
observations of Autolycus, Euclid, and Posidonius, 8, 10;
first catalogues of, 19;
latitude and longitude of, 24, 30;
positions tabulated by Hipparchus, 30;
Tycho Brahe’s catalogue and map of, 42, 44;
stars in Gemini seen through a large telescope, 155;
nebula of Orion, 157;
Orion and its neighbourhood, 156;
double, as defined by telescopes of different power, 162, 164, 167,
167;
distance of stars from the earth, 159;
facilities for observing Orion, its stars, a test for power of
telescopes, 164;
stellar photography, 172, 465, 466, 467, 478;
their rising and setting as measurers of time, 176;
double, measurement of, 359, 361, 362;
spectrum of red star, 433
Star-clusters, double and multiple stars, 351
Star-spectra, from Father Secchi’s observations, 433;
photographs of, 479
Star spectroscopes, at Cambridge (U.S.), 430;
direct vision, 431
Star-time (_see_ Sidereal Time)
Steinheil, improvement of specula, 117
Stellar day, 256
Stereographs of the moon, 465, 466
Sternberg, Tycho Brahe’s Observatory, 38
Stewart (Prof. Balfour), spectrum analysis, 402;
solar photography, 471
Stokes (Prof.), experiments with phosphatic glass, 123;
spectrum analysis, 402, 410
Stone, thermopile at Greenwich, 384
Strontium in the sun, 419
Struve, transit instrument, 285;
double stars, 362;
star magnitudes, 381
Sun, The; in Ptolemy’s system, 3;
first determination of its yearly course, 8, 15;
course in the zodiac, described by Autolycus, 9;
altitude determined by the gnomon, 16, 18;
and the Scarphie, 19, 20;
telescopes for observing, 114;
“mean sun,” 256;
as a telescopic object, 349;
presence of sodium in, 412, 415;
vapour of other metals, 417;
absorption spectrum, 418;
telespectroscopic observations, 436;
of the chromosphere, 437;
sun-storms, 438, 439;
photographs, 459, 469, 470
Sun-dials, 18
Sun-spots observed by Galileo and Scheiner, 78;
examined by the position micrometer, 358;
spectra of, 415, 435
Sunderland time signals, 278
T.
Talcott, zenith telescope designed by, 285
Taurus, its position in the zodiac, 34
Telegraph wires, their application in determining “local time,” 281
TELEPOLARISCOPE, THE (Chap. XXX.), 441-453
Telespectroscope, 426
TELESCOPE, THE (Book II.), 55-172
TELESCOPE, THE EQUATORIAL (Book V.), 293-368
TELESCOPE:—VARIOUS METHODS OF MOUNTING LARGE TELESCOPES (Chap. XX.),
293-327;
refracting, 73-89;
Galilean, 73;
magnifying power of the telescope, 76, 79;
Scheiner’s telescope, 78;
focal length of early telescopes, 79;
achromatic, 86;
reflecting, 100-108;
Gregory’s telescope, 101;
Newton’s, 102;
Cassegrain’s, 103;
Sir W. Herschel’s 103, 108;
Lord Rosse’s, De La Rue’s, Lassell’s, Foucault’s, Grubb’s, 108;
eyepieces, 109-116;
Huyghen’s eyepiece, 110;
Ramsden’s eyepiece, 112;
magnifying power of eyepieces, 116;
lenses and specula, 117-138;
flint glass for lenses, 119;
the “optick tube,” 139-151;
the modern telescope, 152-172;
magnifying and space penetrating power, 154, 155;
illuminating power, 158;
defining power, 160;
reflecting and refracting compared, 170;
permanence of optical qualities, 170;
“telescopic sight,” 219;
Sir Wm. Herschel’s at Slough, 294;
Lord Rosse’s reflector, 294, 311, 312;
refractor on alt-azimuth tripod, 296;
simple equatorial mounting, 298;
the German mounting, 299;
Washington great equatorial, 309;
English mounting, 310;
forked mounting, 310;
Greenwich equatorial, 310;
Melbourne reflector, 312, 313;
Paris reflector, 314;
driving clock, 318;
Newall’s refractor with spectroscope, 427;
De La Rue’s, 459;
Rutherfurd’s, 466;
Newall’s, 459;
Melbourne, 459
Telescope, zenith (_see_ Zenith Telescope)
Temperature, its effect on the pendulum, 187, 193
Terrestrial globe, 23
Thales, his employment of the gnomon, 17
Theodolite, 288
Theodolite, astronomical, 287
Thermometry, 374, 384
Thermopile, 374
Time; first reckoning of, 19;
early measurements, 36, 44, 175;
modern measurement of, 253;
sidereal, solar, and mean, 254, 256
TIME AND SPACE MEASURERS (Book III.), 175-232
Time, Greenwich (_see_ Greenwich Time)
Time, local, 281
Time balls for distributing Greenwich time, 275
Time signals, 278, 281, 283
Timocharis, his observations in the Alexandrian museum, 19
Tourmaline, in polarization of light, 443
TRANSIT CIRCLE, THE (Chap. XVI.), 233-252;
system of wires in eyepiece, 220;
at Greenwich and Cambridge (U.S.), 247, 248, 251;
mode of using, 253, 284
TRANSIT CLOCK, THE (Chap. XVII.), 253-270
Transit instrument, 171, 234, 236, 237;
mode of using, 253;
Römer’s, 284;
Struve’s, 285
Transit of Venus, photographic observations, 475
Trigonometrical tables, first construction of, 17
Tropics, defined by Euclid, 12
Trouvelot, ring of Saturn observed with the Washington refractor, 161
Tube of the telescope, 139-151
Tycho Brahe; astrolabe, 26;
ecliptic astrolabe, 28;
discoveries of, 37-52;
biography of, 37;
list of his instruments, 38;
portrait, 39;
catalogue of stars, 42;
observatory (engraving), 43, 287;
his solar system, 46;
discovery of comet of 1677, 47;
instruments for measuring distances and altitudes of stars, 51;
clocks, 179, 184, 196;
diagonal scale for measuring space, 213;
mural quadrant, 233;
transit circle, 284
U.
United States Naval Observatory, 341
Uranus, as a telescopic object, 351
Uraniberg, Tycho Brahe’s Observatory, 38
V.
Variable stars, 377
Velocity of gases in sun-storms, 440
Venice, ancient clock dials, 257
Venus, in Ptolemy’s system, 3;
in Tycho Brahe’s, 46;
employed by Tycho Brahe in determining longitude, 44;
as a telescopic object, 350;
transit of, instrument used in the expedition of 1874, 236;
photographic observations, 475
Vibrations, ethereal, 373, 401, 410, 449, 450
Vienna, refracting telescope, 141
Villarceau, Yvon, driving clocks, 324
Vega, heat of, 385
Vernal equinox, its position in the constellations, 34
Vernier, the, 214
Vertical circle, Ertel’s, 290
W.
Walther, altitudes measured by, 36
Washington Observatory; great refracting telescope, 302, 309;
flint glass discs, 119;
ring of Saturn seen through it, 161
Watches, detached lever escapement for, 207
Water clocks, 176
Wave-lengths of light of solar gases, 440
Westminster clock-tower, 277
Wheatstone (Sir C.); “chronographic method” of transit observation,
259;
apparatus for controlling clocks, 271
Winlock (Prof.), photographs of the sun, 461
Wires, cross, for circle reading, 212, 216;
system of wires in a transit eyepiece, 220, 234, 257;
in eyepiece of Greenwich transit circle, 246;
wires of the transit instrument, 234
Wire micrometer, 221, 352
Wolfius, correction of chromatic aberration in lenses, 89
Wollaston (Dr.), lines in the solar spectrum, 391;
spectrum analysis, 402, 422
Wyck (Henry de), clock made in 1364 by, 178
Y.
Ys of the transit instrument, 238, 284
Years, first observation of, 5;
determination of their length, 22
Z.
Zenith, zenith sector, zenith telescope, reflex zenith tube, at
Greenwich, 285
Zenith distances, measurement of, 51
Zodiac, first defined, 8, 9;
observations of Euclid, 11, 12;
of Denderah, 7
Zöllner’s astrophotometer, 379
Zero of right ascension, 249
Zinc in the sun, 419
THE END.
LONDON: R. CLAY, SONS, AND TAYLOR, BREAD STREET HILL, E.C.
TRANSCRIBER'S NOTES
1. Silently corrected typographical errors.
2. Retained anachronistic and non-standard spellings as printed.
3. Enclosed italics font in _underscores_.
4. Enclosed bold font in =equals=.
5. Superscripts are denoted by a carat before a single superscript
character or a series of superscripted characters enclosed in
curly braces, e.g. M^r. or M^{ister}.
6. Subscripts are denoted by an underscore before a series of
subscripted characters enclosed in curly braces, e.g. H_{2}O.
End of Project Gutenberg's Stargazing: Past and Present, by J. Norman Lockyer