Abstract: A musical drum 40 which utilizes a principle of physics to achieve an unequal tensioning of the upper head 20 and the lower head 36 without the need of physical attachments or piercing of the drum shell 24. The shell of the drum 24 remains suspended by shell's bearing edge 26, in contact only with the upper and lower heads 20 and 36 respectively. This allows the shell 24 of the drum to resonate freely and unencumbered by any attachments of hardware.

Abstract: An interactive learning system has an interactive control unit that displays competition or quiz questions in multiple-choice or other format. Students using the system enter answer choices on a wireless keypad. A base station connected to the interactive control unit receives answer choices from and communicates with the wireless keypads. The interactive control unit keeps track of the scores for all users, and scores of different groups of users (i.e., classes).A receiver connected to the interactive control unit receives content for competitions by satellite broadcast. The competitions are then stored for later use. The system can conduct competitions from those that have been stored or broadcast, or from those that can be created by individual teachers. Previously broadcast and stored competitions can be played simultaneously by a plurality of remote locations, in a "live competition" setting.

Abstract: An optical interferometer includes a beam splitter 131 having a partially reflecting metallic film 135, 137 adapted to induce phase difference between the transmitted and reflected component beams. It incorporates bidirectional fringe counter which includes a switching circuit to convert coupling with the fringe counter means from ac coupling to dc coupling when the counting rate falls below a predetermined threshold value.

Abstract: A microscope used to investigate the profile of a surface (28) comprises a laser light source (10) from which a beam of radiation passes through a collimator (12) and polarizer (16). Orthogonally polarized components are focused at different positions by a birefringent lens system (22,260). Light reflected by the surface is passed to a photodetector (36) which passes an output signal to an analyzer circuit (not shown).

Abstract: An interference refractometer of the Jamin type includes a beam splitter (14) arranged to split light from a laser source (12) into two partial beams B.sub.M, B.sub.R one of which passes through a test cell (20) adapted to contain a fluid medium. One component (B.sub.R) of the reference partial beam passes through a phase plate which introduces a 90 degree phase shift. The partial beams are detected by photodetectors (38, 44, 46) the outputs of which are coupled to counter circuits (not shown) to provide an output which is independent of changes in the mean level of the optical input signals.

Abstract: In a surface profile interferometer, a test surface is illuminated by a beam having components at orthogonal directions of polarization which are focused at different axial distances so that the areas of intersection with the surface differ by a factor of ten or more, and any phase difference introduced by reflection by the surface is sensed by an interference technique. Alternatively, the beam components are focused at equal distances but are laterally separated.

Abstract: In a Michelson interferometer system arranged to give four output interferograms, two at each orthogonal direction of polarization, the reflection and transmission coefficients of the interferometer beam splitter (10) at the two polarization directions are chosen so that the ratios of the a.c. component to the d.c. component are equal in three of the interferograms. Difference signals derived from the three interferograms are suitable for use with an automatic, reversible, fringe counting system.