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About the Icons Shown in the Homepage

The characters, tools, science, technologies, and histrical symbols regarding "length" are iconized and shown in the homepage. All icons are shown in the homepage and one of them is shown in the other pages on a day-by-day rotating basis.

Their meanings are shown in the table below.

List of icons

Icon	Explanation
	Shape of measuring length by a thumb and an index finger. It is the origin of the chinese character of "shaku" (old unit of length.)
	The old unit of length "cubit" was based on forearm's length.
	Shape of extending both one's arms. This length had been used as the length unit "hiro."
	The unit "feet" originates from the size of one's foot.
	Old chinese character of "do". It has meaning of measuring the size of a seat by right hand. Therefore, "do" has the meaning of "measure, or scale"
	The symbol of the unit of length, "meter."
	The first definition of the meter was one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the Equator to the North Pole.
	The distance from Dunkerque to Barcelona was measured by triangulation along the meridian through Paris.
	The measurement of distance between Dunkerque and Barcelona was divided into northern and southern parts. These two parts of measurement met at Rodez Cathedral.
	The cross section of the prototype of the meter. It is called "Tresca's crosssection" named for the designer.
	Vernier caliper has a vernier scale addition to a main scale to improve measurement resolution.
	Micrometer has precise screw inside. The screw amplify a small distance into the rotations of the screw.
	Gauge blocks are widely used practical length standard. It was developed by Carl Edvard Johansson. The length of a gauge block is defined as the distance between opposing lapped surfaces.
	Scales
	The second definition of the meter was based on the wavelength of Kr lamp.
	The present definition of the meter is based on the speed of the light, "the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second."
	Two or more coherent light waves generate interference fringes. The period of the interference fringe according to the path length is equal to the wavelength of the light. Interference is widely used in precise length measurements.
	Laser has high coherency and offen used in optical length measurements.
	Refractive index of material effect on the length the light undergoes. Most of the material has remarkably dependence of the refractive index on wavelength of the light. We should consider the refractive index of the air in the precise length measurements.
	Electronic distance meter
	Optical frequency comb has wide spectral range and comb-shaped optical frequency modes. These modes have equal interval and can be used as a scale in optical frequency. Present Japanese national standard of length is the optical frequency comb.