Camera Rotate to mouse position

Hi guys, I need some help to make this code work, I’m making a point and click game ad I want that in some cases the camera act something like an human head (first person view)
I want the camera to rotate where the mouse is looking at making it at the center of the screen, so far I’ve made this code:

var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
	var hit : RaycastHit;
	if (Physics.Raycast (ray, hit, 1200)) {
    	Debug.DrawLine (ray.origin, hit.point);
    	camera_.LookAt(hit.point);
	}

The problem with this code is that the camera spins on itself because the mouse is always pointing to a new world position, how can I make the mouse to move smooth to the center?

Thanks for the help

var targetRotation : Quaternion; //Script variable
var speed = 2.0;

   ...

   var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
   var hit : RaycastHit;
   if (Physics.Raycast (ray, hit, 1200)) {
       Debug.DrawLine (ray.origin, hit.point);
        targetRotation = Quaternion.LookRotation(hit.point - camera_.transform.position);

    }

   ...

   function LateUpdate() {
          transform.rotation = Quaternion.Slerp(transform.rotation, targetRotation, Time.deltaTime * speed);
   }

A very meaningful event, I hope everything will go well temple run