A cradle for supporting and retaining a tablet computer having four sides, a front, and a back. In one embodiment a connector frame has a first end disposed against the first circuit board and defining a first wall, and an opposing second end disposed against the second circuit board and defining a second wall generally parallel with the first wall. The ultrasound signal provided by the transmitter has a bandwidth of more than 80%, preferably more than 100% of the nominal frequency of the ultrasound signal provided by the transmitter. The polarizer transmits light having polarization directions obtained by inverting the polarization directions set by the polarization setting element with respect to the straight line parallel to the first direction, respectively. The current mirror circuit is connected to the divider circuit. At least some of the hard bodies have a continuous recess, wherein the recess extends in the radial direction of the grinding roller.