void zoom(num distance)

Zooms in or out depending if the distance is positive or negativ.

Source 

void zoom(num distance) {
  // Move the camera origin along the forward direction
  num newCenterX = center.x + distance * forward.x;
  num newCenterY = center.y + distance * forward.y;
  num newCenterZ = center.z + distance * forward.z;

  center = new Point3(newCenterX, newCenterY, newCenterZ);
}