The three acts of Brave New World can be marked roughly by the changes in setting. The first act, therefore, runs from Chapter One up though Part Two of Chapter Six, and it takes place in the civilized world of London.
It follows that Act II covers Bernard and Lenina's time in the Savage Reservation: Chapters Seven, Eight, and Nine. It is during Act II that we have our protagonist switch from Bernard to John.
The last act begins right around the time John is puking his guts out in response to the new world. The transition from Act II is his shifting opinion of the nature of society, from admiration to disgust.