code: function(brightness, callback) {
      if (brightness < 0 || brightness > 15) {
        throw "Invalid brightness number";
      }
      this._shiftOut(MAX7219._Registers.Intensity, brightness, callback);
    }
docs: Sets digital control of display brightness.
  
  @param number brightness
  The brightness from 0 (dimmest) up to and including 15 (brightest).
  
  @param function callback [optional]
  Invoked once the write to the SPI device finishes.