Fluctuations in probability distribution are used to study the influen
ce of a weak periodic perturbation on a chaotic system. A detailed ana
lysis of one very long time series reveals a great sensitivity to an e
xternal perturbation. The case of a large amplitude stochastic perturb
ation is also discussed. It is shown that the fractal geometry of an e
xperimental attractor may be visualized up to length scales smaller th
an the noise amplitude.