Flicker thresholds of harmonically modulated light with variable frequency and flash thresholds of square flashes of variable duration are compared quantitatively. The experiments were carried out with the same subjects under the same conditions and with the same equipment over an intensity range of five decades. Characteristic quantities for sensitivity and inertia are defined for both types of measurement. Sensitivity S for flicker and sensitivity F for flashes are found to have a constant ratio over the entire background intensity range. The product of the cut-off frequency with flicker and the critical duration with flashes is also found to be constant.