Reference line The general index line refers to the line system engraved on the imaging plane of optical instruments as observation reference, which is called scale line, also called dividing line. The telescope sight of a gun (usually called a mirror frame) is probably the most relevant device for the crosshair. Movies and media often use the crosshair as a dramatic device, which makes the cross have a wide cultural exposure.
A scale line (from Latin network, meaning "net"), also known as a scale line (from Latin character string, meaning "grid"), is a thin line or fiber net in the eyepiece of a sighting device, such as the screen of a telescope, microscope or oscilloscope. Today, carved lines or embedded optical fibers can be replaced by computer-generated images superimposed on the screen or eyepiece. These two terms can be used to describe any group of lines used for optical measurement, but in modern masks, they are most commonly used for hand holding, etc., while scales are more widely used for oscilloscope covers and similar functions.
There are many varieties of crosshairs; This paper focuses on a simple marker line: the crosshair. Crosshairs are usually represented as cross lines in the shape of a cross, "+",although there are many variations, including points, columns, circles, scales, herringbone patterns or combinations of these. The most common telescopic sight used to aim at firearms, crosshairs are also common in optical instruments used in astronomy and measurement, and are also called precision pointers in graphical user interfaces. It is said that this mark was invented by robert hooke and can be traced back to17th century. Another inventor candidate is amateur astronomer William gascoyne.