Tap one measure to pick it up, then tap another to pour into it. It pours until the source is empty or the target is full: there's no stopping halfway, and there's no tap here. Whatever is in the flask is all there is.
The doctor has prescribed two ounces of tincture and not a drop more: three ounces and she dies. There's no scale or graduated flask in the back room, only three unequal tin measures — eleven, seven, and four ounces — and the flask filled to the brim. You can't pour a half-measure or throw anything away: only pass it from one to another until one is empty or the other full.
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