throbbing light machine

From "Sonnets From an Ungrafted Tree," Edna St. Vincent Millay:

IX.

Not over-kind nor over-quick in study
Nor skilled in sports nor beautiful was he,
Who had come into her life when anybody
Would have been welcome, so in need was she.
They had become acquainted in this way:
He flashed a mirror in her eyes at school;
By which he was distinguished; from that day
They went about together, as a rule.
She told, in secret and with whispering,
How he had flashed a mirror in her eyes;
And as she told, it struck her with surprise
That this was not so wonderful a thing.
But what's the odds? — It's pretty nice to know
You've got a friend to keep you company everywhere you go.