.]. .Iadd.enhancing .Iaddend.the portion of the signals representing the particular fluorescence .Iadd.during the particular time period .Iaddend.relative to the portion of the signals representing the .[.remaining.]. .Iadd.background .Iaddend.fluorescence .Iadd.during the particular time period.Iaddend., and the data processing means including first means for differentiating the signals produced