The virtual or drawn line created by the straightedge is called an " index line " or " isopleth ".
3.
It is adjusted by hand, even with Arctic mittens, simply by rotating the top of the fuze clockwise until a 3-letter engraving is above an index line.
4.
EuropeanTime throughout Europe was electrically coordinated in the late 19th and the 20th century ny noting when stars passed the index line of a telescope at that observatory set on an axis along that Prime Meridian.
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