What was the strangest thing about the way the first spirit looked?

The first spirit is the Ghost of Christmas past. His purpose is to remind Scrooge of his childhood..... the boy he once was, the feelings he had as a child.

But the strangest thing about it was, that from the crown of its head there sprung a bright clear jet of light, by which all this was visible; and which was doubtless the occasion of its using, in its duller moments, a great extinguisher for a cap, which it now held under its arm.