The spooky cold that Francisco describes as he and Barnardo exchange posts thoroughly sets the mood of the play, which Yale Professor Maynard Mack describes as "mysterious and equivocal, a mixture of bright surfaces and dark forces where what seems both is and is not."
This scene shows very clearly the problem of discerning between appearance and reality. The Ghost appears, but is it really there? If it is there, is it really a devil assuming the king's regal shape and garments? Distinguishing between truth and illusion is the focal dilemma of Act I and will challenge Hamlet right up to the play's turning point in Scene 4 of Act IV. Barnardo's questioning of Francisco introduces the idea that Hamlet's world is upside-down. Protocol dictates that Francisco should question the newcomer, but here the interloper questions the guard. Francisco's response reinforces the sense of malaise. His "sickness at heart" prefigures the tension of the ensuing tragedy, while the changing of the guard mirrors the tenuousness of the political climate of Denmark — the transition from one king to another and the arrival of the Prince whose rightful place on the throne has been usurped.
In this first scene of Hamlet, Shakespeare introduces a set of mirrors that will pervade throughout. Fortinbras, a young man whose father has been defeated by a foe and whose obligation is to avenge that father's death and reclaim the conquered properties, serves as a foil for Hamlet. Several characters will reflect Hamlet, but Fortinbras is the first to be named in the play in whom we see a likeness to the Prince of Denmark.
