As I returned her intent look--which I could do unobserved, since my face, reflected in the mirror, was in deep shadow--I suddenly perceived that that steady gaze engaged her right eye only; the other eye was looking sharply towards her left shoulder.

In short, she had a divergent squint of the left eye.
I put down my cup with a thrill of amazement and a sudden surging up of suspicion and alarm.

An instant's reflection reminded me that when she had spoken to me a few moments before, both her eyes had looked into mine without the slightest trace of a squint.

My thoughts flew back to the lump of sugar, to the unguarded milk-jug and the draught of tea that I had already swallowed; and, hardly knowing what I intended, I started to my feet and turned to confront her.

But as I rose, she snatched up her change and darted from the shop.