首楞嚴義疏注經 4.454

4.454

前二句總告，如眼下正示。左右旁觀三分之二者，舉一方三百方中二百，全近維二五十，一方既爾，餘皆例然。今左右觀，各得二百全分，近維二五十，共成五百，及全明三百，都成八百。三分言功者，見三方也，餘皆可知。

The first two lines are a general address; from "As when the eye" onward directly shows it. "Looking to the left and right sides yields two-thirds" means taking, out of three hundred for one direction, two hundred within the directions, plus fifty each for the two intermediate directions close by; since one direction is like this, the rest follow by analogy. Now, looking left and right, each gets a full two hundred, plus fifty each for the two close intermediate directions, together making five hundred; together with the fully bright three hundred, they total eight hundred. "Three parts express the function" refers to seeing three directions; the rest is all understandable.