Abstract:
The most conspicuous and readily observed feature of a normal sunspot is a dark central umbra surrounded by a less dark penumbral fringe which, under good seeing conditions, appears to be composed of converging filaments directed radially inwards. Secchi1 concluded from his visual observations that the penumbra is brighter at its inner edge, that is, towards the periphery of the umbra and darker at its outer edge. This feature is clearly shown in his drawings of sunspots.
