A practical experiment using coincidence techniques is suggested to test the validity of the following concepts:(1) wave packet reduction and(2) the measurement-uncertainty principle for position and momentum. The suggested experiment uses the time-of-flight method to determine an electron's momentum and a coincident photon, emitted from a system excited by the electron, to determine its initial position. It is shown that this method does constitute a simultaneous measurement of position and momentum for a single system. Also, it is pointed out that (...) the traditional statement of the measurementuncertainty principle must be slightly modified even within the Copenhagen interpretation of quantum mechanics. (shrink)