Caleb "stands behind the microscope, his eye pressed to the eyepiece" (45.1). See, this is why Caleb is Erudite: he knows not to press his forehead against the eyepiece.
Caleb is also memorizing the activation code, which will enable the memory serum weapon.
Because Caleb knows his time is limited, he decides to play a little game of Candor with Tris. It's like Truth or Dare but without any dares.
Caleb asks Tris point blank if she's actually forgiven him.
Tris says yes, and she wants to know why he's actually sacrificing himself.
Caleb says it's the only way for him to escape guilt. Tris was afraid that's what he'd say (maybe because his thinking is so much like hers… well, except that he skips the whole "death is awesome" phase she went through.)
Caleb's time ends up being more limited than he thought. An emergency lockdown goes into effect, and Matthew and Tris realize that they have to act now.