  We present a straightforward method for measuring the fluids' relative
viscosity via a simple graphical analysis of the normalised position
autocorrelation function of an optically trapped bead, without the need of
embarking on laborious calculations. The advantages of the proposed
microrheology method become evident, for instance, when it is adopted for
measuring the molecular weight of rare or precious materials by means of their
intrinsic viscosity. The proposed method has been validated by direct
comparison with conventional bulk rheology methods.
