A trend in computing environments today is to move towards more natural interaction, another is to make hardware invisible to the user. Both these ideas converge into ubiquitous computing - the Digital Desk is an example of this idea. In this paper we concentrate on an input device for the Digital Desk, namely the users fingertip, which is made to act like a mouse. Tracking such an input device is common to a number of augmented reality environments and involves vision and motion analysis. However, previous attempts have focused more on the vision aspect of tracking general objects than on using the information already known about the users hand, which is the approach taken here. We adopted the goal of tracking the users fingertip as fast as possible in real-time so the system could be compared with other input devices, using models such as Fitts Law. Our system is shown to comply with the law adequately.