The following were measured: the thresholds of pairs of incremental and decremental flashes as a function of their duration, those of doublets as a function of the flash interval and those of incremental flashes over a very large range of durations. The measurements constitute a coherent test of a model based on general system properties. The incremental and decremental threshold characteristics do not differ significantly, from which it is concluded that the excursion of the system responses to the flashes in the positive and negative direction must be approximately equally large if the flashes are to be detected. The doublet characteristics confirm that the response to short flashes contains a definite second phase. At low and high levels, however, they are practically isomorphous, which represents a deviation from the characteristics calculated from the De Lange flicker fusion curves. Measurements over a large range of durations show that under certain conditions the threshold characteristics of rectangular increments display a minimum and that they are remarkably isomorphous at low and high levels. This minimum was also not predicted from the De Lange characteristic. A refinement of the model introduced in order to explain these results yields a transmission curve which differs from the De Lange characteristics in the direction of greater attenuation at the very low frequencies and which, on the basis of the results referred to above, is also isomorphous for the various background levels.