Main Meridian

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[Regular astigmatism]
In the case of regular astigmatism, there is a direction (meridian) in which refractive power is strongest and a direction (meridian) in which refractive power is weakest. These are called the principal meridian and the weak meridian, respectively, and are perpendicular to each other. When viewed along each of the principal meridians, the light entering the eye is focused as a line. …

*Some of the terminology explanations that refer to the "principal meridian" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

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［正乱視］
　正乱視の場合には屈折力のもっとも強い方向(経線)と，もっとも弱い方向(経線)とがみられる。これらをそれぞれ強主経線および弱主経線といい，互いに直交する。強弱主経線ごとにみれば眼に入る光は線となって集まる。…

※「強主経線」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報