function createTracker(){
                for(var i = 0; i < molecules.length; i++)
                    var trackable2DObject = new AR.Trackable2DObject(leftTracker, molecules[i], { onEnterFieldOfVision: toSeeMolecule(molecules[i]), onExitFieldOfVision: toUnseeMolecule(molecules[i])});
            }

            // IR functions //

            function toSeeMolecule (molecule)
            {
                return function () {
                    appendStatus("Recognized " + molecule);

                    loadMolecule("models/molecules/" + molecule + ".pdb");
                    if(!leftOccupied) nMoleculesShown++;
                    appendStatus(nMoleculesShown);
                    leftOccupied = true;
                };
            }

            function toUnseeMolecule (molecule)
            {
                return function () {
                    appendStatus(molecule + " is gone but I'm keeping the 3D around!");
                };
            }