Because one does not stand on the Bridge of Sighs but in it, for it is merely dark passages lit by gratings.

But to stand on the Ponte di Paglia on the Riva and gaze up the sombre Rio del Palazzo with the famous arch poised high over it is one of the first duties of all visitors to Venice and a very memorable experience.
Lastly, the horrible cells (which cost half a lira more), upon which and the damp sinister rooms where the place of execution and oubliette were situated, a saturnine custodian says all that is necessary.

Let me, however, quote a warning from the little Venetian guide-book: "Everybody to whom are pointed out the prisons to which Carmagnola, Jacopo Foscari, Antonio Foscarini, etc., were confined, will easily understand that such indications cannot be true at all.".