What does the following paragraph convey?“Over the prison

What does the following paragraph convey?“Over the prisoner’s head there was a mirror, to throw the light down upon him.Crowds of the wicked and the wretched had been reflected in it, and had passedfrom its surface and this earth’s together. Haunted in a most ghastly mannerthat abominable place would have been, if the glass could ever have renderedback its reflections, as the ocean is one day to give up its dead.”