1. Abdelatif, T. E., Lites, B. W., and Thomas, J. H.: 1986, Astrophys. J. 311, 1015.
2. Balthasar, H. and Wiehr, E.: 1989, in O. v.d. L�he (ed.), Tenth Sacramento Peak Summer Workshop ?High Spatial Resolution Solar Observations? (in press).
3. Balthasar, H., K�veler, G., and Wiehr, E.: 1987, Solar Phys. 112, 37.
4. Beckers, J. M. and Schr�ter, E. H.: 1969, Solar Phys. 10, 284.
5. Braun, D. C., Duvall, T. L., and LaBonte, B. J.: 1988, Astrophys. J. 335, 1015.