Adjustment of the sextant consists of checking and aligning all the optical elements to eliminate " index correction ".
2.
The instrument may have an error, IC or index correction ( See article on adjusting a sextant ).
3.
Index correction should be checked, using the horizon or more preferably a star, each time the sextant is used.
4.
These refractive index corrections can be found more accurately by adding frequencies, for example, frequencies at which propagation is sensitive to the presence of water vapor.
5.
A " refractive index correction " is made to relate the medium used to the reference vacuum, taken in SI units to be the classical vacuum.
6.
By implementing a refractive index correction such as this, an approximate realisation of the metre can be implemented in air, for example, using the formulation of the metre as wavelengths of helium-neon laser light in vacuum, and converting the wavelengths in a vacuum to wavelengths in air.
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