若角膜呈榄球形状，即其弧度呈不规则，水平光线将聚焦在和垂直光线不同的焦点，这便是散光。
Astigmatism occurs if the cornea is shaped like a rugby its curvature is not uniform. As a result horizontal lines are focused to a different point from vertical lines.
虹膜眼睛中一层带色素的，球形的可伸缩的薄膜，处于角膜和晶状体之间，为。
The pigmented, round, contractile membrane of the eye, suspended between the cornea and lens and perforated by the pupil. It regulates the amount of light entering the eye.
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