1. Brusa, R.W. and Fr�hlich, C.: 1986,Appl. Optics 25, 4173.
2. Crommelynck,D., Domingo, V.: 1993,Sol. Phys., submitted.
3. Crommelynck,D., Domingo, V.,Fichot, A.,Fr�hlich, C., Penelle, B., Romero J., and Wehrli Ch.: 1993,Metrologia 30, in press.
4. Fr�hlich, C., Pap, J., Hudson H.S.: 1993,Sol.Phys., submitted.
5. Koopmas, L.H.: 1974,The Spectral Analysis of Time Series, Academic Press: New York.