Arduino attachinterrupt in library

#ifndef tl

#define tl

#if (ARDUINO >=100)

  #include "Arduino.h"

#else

  #include "WProgram.h"

#endif

class TestLib{

  public:

    // Constructor 

    TestLib(int i_pin=0);

    // Methods

    int _measure(int c_interrupt_delay, int i_measure_amount);

  private:

    volatile unsigned int i_pulse;

    int i_measure_pin;

    void _counter();

};

#endif

#include "capSensor.h"

TestLib::TestLib(int i_pin){

  // Anything you need when instantiating your object goes here

  i_measure_pin = i_pin;

}

// Pretend this is one or more complex and involved functions you have written

int TestLib::_measure(int c_interrupt_delay, int i_measure_amount){

  volatile unsigned int i_differenz_puls = 0;

  delay(1000);                                        // Sensor start toleranz delay

  for(int i = 0; i < i_measure_amount; i++)

  {

    delay(500);

    i_pulse = 0;                                      // Anzahl der Pulse auf 0 setzen

    attachInterrupt(0, TestLib::_counter, RISING);    // Interrupt aktivieren: Steigende Flanken

    delay(c_interrupt_delay);

    detachInterrupt(0);                     // Interrupt deaktivieren

    i_differenz_puls += i_pulse;

  }

  i_differenz_puls = i_differenz_puls / 10;

  return i_differenz_puls;

}

// Private method for this class

void TestLib::_counter(){

  i_pulse++;

}

/var/folders/19/qr88mkd91kx47d2l25csn_5m0000gn/T/arduino_build_100013/sketch/capSensor.cpp: In member function 'int TestLib::_measure(int, int)':

capSensor.cpp:19: error: invalid use of non-static member function

     attachInterrupt(0, TestLib::_counter, RISING);    // Interrupt aktivieren: Steigende Flanken

                                                 ^

exit status 1

invalid use of non-static member function