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A distinct structural form of an element, a common example is in carbon where two allotropes are diamond and graphite.
armillary sphere
A skeletal celestial sphere comprised of rings that represent the tropics, equator, zodiac and occasionally the arctics.
azimuth tables
When navigating by the stars, the azimuth of a celestial body is its direction (bearing) from the observer, measured clockwise from due North. An azimuth table allows the azimuth to be calculated for a particular time and latitude.


Beagle voyage
The circumnavigation of the globe (1831-1836) during which Charles Darwin examined a large number of specimens, both biological and geological. The importance to Darwin of the voyage cannot be overstated; much of the work that informed and determined his theory of evolution was carried on board the Beagle.
Bletchley Park
Primary location of the United Kingdom's wartime effort to decipher the coded communications of the Axis Powers.
bull's eye condenser
A bi-convex or plano-convex lens which focuses light directly onto the specimen, and is used to illuminate opaque objects.


celestial sphere
A celestial sphere is an imaginary sphere showing the layout of the stars and other bodies, as viewed from the Earth. The surface of a celestial globe represents the celestial sphere.
chromatic aberration
An irregularity in the refraction of light through lenses that occurs because white light is made up of a spectrum of colours, which each have different wavelengths. As each wavelength is bent through the lens at a different angle, it causes fringes of colours to appear around the image. See also spherical aberration. G 
chromatic scale
A series of 12 notes, ascending or descending in pitch G , each of which is a semitone away from its neighbour. On the piano, this scale is played by pressing each black and white key in series from one end to the other of the keyboard.
The process by which a liquid changes into a thickened, curd-like, insoluble state as it undergoes a chemical reaction.
concert pitch
The name given to the standard tone to which musicians tune their instruments. Normally when an orchestra assembles for a performance, the orchestra tunes to concert A, given by the oboe. After centuries of dispute, the international standard for concert A was set at 440 vibrations per second (or Hz G ) in 1926.
The phenomenon where two or more musical tones sound good together. It is partly a subjective matter as to whether tones are consonant together, but it has been observed since ancient Greece that consonant tones bear certain simple mathematical relationships to each other. For example, pairs of notes instantiating the perfect octave G  and perfect fifth G  in so called 'just intonation' (examples of the most perfect consonances) stand in frequency ratios of 2:1 and 3:2 respectively.
Copernican system
The Copernican system takes its name from Nicolaus Copernicus, who proposed that the Earth and the other planets revolved around the Sun. This theory effectively replaced the Ptolemaic System G  in the 200 years or so after it was proposed in 1543.
The mapping and description of both heavenly and earthly features, emphasising the relation between the two.
Culpeper-type microscope
A style of compound microscope designed by Edmund Culpeper. This distinctive upright design is focussed by moving the draw-tube in and out, and was the first design to incorporate a substage mirror G  as standard.


equal temperament
The system of keyboard tuning where the 12 semitones of the octave are divided equally, thus ensuring that every corresponding musical interval across the keyboard is also equal (e.g., every major third is the same size independent of musical key).


Fender-Rhodes electric piano
An electromechanical keyboard instrument, which was developed around the 1950s and marketed in the USA in 1965. It became very influential in the sphere of jazz and jazz fusion in the hands of such musicians as Joe Zawinul. Whereas a piano uses strings struck by hammers to produce musical sound, the Fender Rhodes uses asymmetrical tuning forks. One half of the fork is more flexible and gives the note its distinctive attack, and the other is stiffer and more resonant, giving the note its sustain. The vibrations of the forks are turned into electrical signals by pickups, such as you find in electric guitars, and then amplified.
The rate at which something occurs. The Olympic games, for example, have a frequency of once every four years. In music, frequency refers to the number of vibrations per second of a sound wave. Concert A, for example, is defined as 440 vibrations per second (or Hz G ).


glass armonica
A musical instrument invented by the American polymath Benjamin Franklin (1706-1790), comprising 37 tuned glass bowls rotating on a spindle. The instrument is played by touching the rims of the bowls with wet fingertips, much as one can play notes on wineglasses. The instrument was very popular during the 18th century, with composers such as Mozart writing for it. But it fell out of favor as rumors spread that it adversely affected one's health and even caused madness.
The triangular pieces that combine to form the surface of a celestial or terrestrial globe.
Gregorian calendar
A modification of the Julian calendar G  that was adopted in Britain in 1752. The modifications corrected for errors and brought the calendar in line with the seasons.


Harmony exists where two or more musical voices combine to produce a series of consonant (see consonance G ) or dissonant progressions, usually designed to produce some kind of satisfying emotional or dramatic effect. Whereas 'consonance' applies to static arrangements of notes, harmony usually implies a movement in some 'logical' progression with tension (dissonance) and resolution.
horizon ring
A fixed circle that does not revolve with the globe, but instead is part of the stand. The horizon ring represents an observer's horizon: all points on the sphere above the horizon ring are visible.
(Pronounced "Hertz".) The international standard unit of frequency G , named after the German scientist Heinrich Hertz (1857-1894) who carried out important work concerning electromagnetic radiation. 1 Hz is one cycle per second.


Julian calendar
Calendar system introduced by Julius Caesar, where the year is divided into 365 days, with a leap year of 366 days every four years.


The distance north or south of the equator for a particular place on the earth's surface.
live cage
A microscope accessory that allow samples of water or live specimens to be enclosed and observed.
The distance east or west of the Prime Meridian G  for a particular place on the earth's surface.


magic lantern
A device for projecting an image onto the wall.
Manhatten Project
Initiated during the 2nd World War, the Manhatten Project was the United States' programme responsible for developing and producing the world's first nuclear weapons.
meridian ring
A full circle or semi-circle on a globe that passes through the earth's north and south poles (on a terrestrial globe), or the celestial poles (on a celestial globe). The meridian ring does not revolve with the globe, but instead holds the globe in place. On many celestial globes the meridian ring, also known as a meridian circle, can be adjusted to show the appearance of the sky at a particular latitude G .
The measurement of microscopic objects; the use of a micrometer (a device for measuring microscopic objects).
When light rays do not meet or focus in one point.
The microspectroscope is a version of a spectroscope G  that can be replace the eyepiece in a compound microscope.


natural frequency
If you flick the rim of a wineglass it will produce a note. Flick it again and it will produce the same note. The glass is vibrating at its natural frequency. We might simply say that the natural frequency of an object is the frequency G  at which it most naturally vibrates when struck, plucked or bowed etc.
natural philosophy
The study of natural phenomena. In the early-modern period "science" had various meanings, but until the mid-18th century it did not specifically refer to the study of nature; hence "natural philosophy" or "natural science" is used to describe those who studied natural phenomena.
Newtonian physics
A system of physics, based upon the "Laws of Motion" devised by Isaac Newton. Now contrasted with quantum and relativistic physics.


perfect fifth
The musical interval corresponding to a frequency ratio of 3:2. In practice modern instruments have to narrow their fifths slightly in order to preserve the sound of the other musical intervals such as thirds, fourths and octaves. A fifth can be heard at the piano by playing any interval seven semitones wide (e.g., from C to G). It is called a fifth since it corresponds to the interval between the first and fifth notes of the major scale.
perfect octave
The musical interval corresponding to a frequency ratio of 2:1. An octave can be hear at the piano by playing a any interval 12 semitones wide.
The differing amounts of a planet or the Moon that are visible, e.g. full, new, crescent.
If frequency G  is the objective measure of the rate of vibration of a sound source, pitch is the subjective or perceived correlate. Although there is generally a close correspondence between frequency and pitch, there are cases where the two come apart. For example, most laptop speakers are too small to reproduce low frequency bass sounds, but when listening to music we nevertheless perceive bass sounds that would correspond to those missing low frequencies. This is explained by the brain 'filling in' the missing information based on its correct detection of other frequencies present in the signal.
A flat representation of the celestial sphere G .
The condition in which the vibrations of light are restriced to a single direction.
Prime Meridian
The line defined as being of zero longitude G . By international convention, the Prime Meridian passes through the Royal Observatory in Greenwich, London.
Ptolemaic system
The astronomical theory that posits a stationary Earth with the Sun, stars and other planets revolving around it. It derives its name Ptolemy of Alexandria who lived in the second century AD. See also Copernican system G .


reflecting telescope
A telescope that magnifies using mirrors rather than lenses. Light enters and is reflected twice before it enters the eyepiece. See also refracting telescope. G 
refracting telescope
A telescope that magnifies using lenses rather than mirrors. The images passes through convex lenses and then enters the eyepiece. See also reflecting telescope. G 
Resonance can be observed in the following situation. When two adjacent guitar strings are tuned to the same note and one string is plucked the air vibrations caused by the first string will make the second string vibrate 'in sympathy': if you stop the first string the second will continue to sound. Resonance can also be used to amplify vibrations. Imagine pushing a child on a swing, for example. When you first push, the movements are small, but as you continue to give small regular pushes, each time just as the swing begins its motion away from you, the amplitude of the movement becomes larger and larger - the swing is resonating with your regular pushing motion.
rhumb line
A navigational term for a straight line path defined by an initial bearing.
Royal Society
The Royal Society is a society incorporated by Charles II in 1662, for the pursuit and advancement of the physical sciences (definition from the Oxford English Dictionary).


The record of the motion of an earthquake, produced by a seismograph G . Seismograms can either be recorded on paper, or increasingly are stored as digital data on computers.
An instrument for automatically recording earthquake motion. A simple seismograph consists of a very heavy weight with a pen attached and some paper to trace the motion. When an earthquake occurs, the weight and pen will stay still, due to the large mass of the weight. As the piece of paper will move along with the ground, the pen will trace out the motion on the paper. The record of the earthquake motion is called a seismogram G .
solar microscope
A type of microscope that illuminates the object with sunlight gathered from a large mirror. The image is then projected onto a wall or clear surface so that many observers can view it.
A spectroscope is used to analyse the spectrum of light by splitting it up into its component colours using prisms.
The band of colours that white light is split up into using a prism or diffraction grating.
spherical aberration
The failure of a spherical mirror or lens to make light rays meet in one point. See also chromatic aberration. G 
standing wave
If you throw a stone into a pond waves are formed which can be seen to travel outwards from the centre. A standing wave, by contrast, is a wave that does not travel, for instance in the waves commonly formed on a guitar string. In this case, when you initially pluck the string two waves are formed that propagate in opposite directions, are then reflected at each end and recombine to form the standing wave pattern. On a Chladni plate, the initial excitement with the violin bow sends traveling waves across the plate which are reflected from the edges and then combine with each other to form the complicated standing wave patterns.
substage mirror
A mirror made from either glass or polished metal, which reflects light up to the specimen. This form of illumination can only be used for trasparent specimens; if the specimen is opaque it must be lit from above, sometimes with the aid of a bull's-eye condenser. G 


the Doctrine of the Sphere
Teachings extracted from the Tractatus de Spera by John de Sacrobosco, based on trigonometrical relationships between lines on the celestial sphere.
the Enlightenment
A movement centred in 18th century Europe that was characterised by philosophical and political ideals such as progress, liberty and empiricism.
the Renaissance
Beginning in Italy before spreading across much of the rest of Europe, the Renaissance was a period between the 14th and 17th century characterised by a renewed attention to ancient Greek and Roman ideas.
Timbre, or tone colour, refers to the particular quality of a musical note. For example, middle C played on a piano has a different timbre from middle C played on a French horn. The timbre of a note is determined by the particular combination and intensity of harmonic partials present in the signal.
Tonometers are frequency G  standards used for measurement and calibration. The first tonometer was built by Johann Scheibler (1777-1837) in 1834 and comprised 56 tuning forks at intervals of 4 vibrations per second. Tonometers were also built by Rudolph Koenig (1832-1901) using tuning forks, but also an alternative design using tuned metal bars was very common.
When either Venus or Mercury passes in front of the Sun.


undulatory theory
Also known as the wave theory of light, it is the theory that light is due to vibrations travelling through a medium.


van der Waals radius
The van der Waals radius of an atom is the radius of an imaginary hard sphere that can be used to model the atom for many purposes. Van der Waals radii are determined from measurements of atomic spacing between pairs of unbonded atoms in crystals.
vibration modes
These are simply the different ways in which an object can vibrate. For instance guitar sting can vibrate in its fundamental mode where the ends of the string are stationary and every other part of the string is in motion from side to side. At the next mode, the centre of the string can be stationary, forming a node, as the top and bottom halves of the string move in opposite directions. To get this mode simply pluck the string in the middle and then gently touch the middle with your finger. You will hear the 'bottom fall out' of the note leaving the octave higher sounding.
A volvelle is a moveable paper disc which when rotated can be used as a tool for performing calculations. They were common in astronomical books of the 16th century and were used as calendars or to show phenomena such as lunar phases or the position of the planets or stars.
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