Positioning RayCast

How do I find the center of the screen.

var ray = mainCamera.ScreenPointToRay (Vector3(300,300,0.2));

The 300,300 is what I can’t seem to figure out. I want to center the character.

original

function DragObject (distance : float)
{
	var oldDrag = springJoint.connectedBody.drag;
	var oldAngularDrag = springJoint.connectedBody.angularDrag;
	springJoint.connectedBody.drag = drag;
	springJoint.connectedBody.angularDrag = angularDrag;
	var mainCamera = FindCamera();
	while (Input.GetMouseButton (0))
	{
		var ray = mainCamera.ScreenPointToRay (Vector3(300,300,0.2));
		springJoint.transform.position = ray.GetPoint(distance);
		yield;
	}
	if (springJoint.connectedBody)
	{
		springJoint.connectedBody.drag = oldDrag;
		springJoint.connectedBody.angularDrag = oldAngularDrag;
		springJoint.connectedBody = null;
	}
}

function FindCamera ()
{
	if (camera)
		return camera;
	else
		return Camera.main;
}

The center of the screen is located at Vector2 (Screen.width/2, Screen.height/2).