Hamlet 3.4 Language: "Act" and "Index"

The transcript discusses how Hamlet raises his voice to Gertrude, and the use of the word 'index' as a metaphor for a list of images. It also explores Gertrude's pun on the word 'act', relating it to the structure of Shakespeare's plays, which are divided into acts. The term 'index' is further explained as a prelude or introduction to the act that follows.

In what way does the text suggest that Gertrude is punning on the word 'act'?

What does the phrase 'introduction to what follows' imply in the context of the play?