This is the process of etching with which Rembrandt did his matchless work.

He varnished a copper plate with black varnish.

With a needle he scratched upon it his design, which looked light where the needle had revealed the copper.

Then the whole plate was put into a bath of acid, which ate away the metal, and so bit into the lines, but had no effect upon the varnish.

When he wanted the lines to be blacker in certain places, he had to varnish the whole rest of the plate again, and put it back into the bath of acid.