feature the hold feature could also be automatically activated by the action of a separate proximity sensor on the bottom of the mouse this is what is shown in fig4 where a shouldered aperture 26 in the base 6 receives a shouldered plunger 25 made captive by the lever arm of a switch 28 above the switch 28 is activated by movement of the plunger 25 such that when the plunger moves significantly in the direction of arrow 27 the hold feature is activated the exact nature of the separate proximity sensor is a matter of choice and while it could be a simple as the micro switch 28 operated by the weight of the mouse through the plunger 25 other non mechanical methods are also possible yet another way to automatically activate and deactivate the hold feature is to examine the nature of the digitized data of the array 10 of photo detectors when the outputs of the photo detectors become sufficiently uniform it may be surmised that there is no longer an image with variations projected onto the array 10 of photo detectors this uniformity will reveal itself by producing a correlation surface that is flat or nearly so rather than separately detecting uniform levels which would use hardware not otherwise present we prefer instead to examine the shape of the correlation surface which surface we need for other reasons anyway the most probable cause of a flat correlation surface is that the mouse.