Richard, thank you. I managed to get it right this time. The reason it didn’t work at first, is that the visibility of the mask was switched off (by default?). I saw this when I selected the ‘channels’ tab in the layers palette. The little visiblity-eye was not there. After I had turned it on everything worked as you described.
I think I need a good book because Photoshop doesn’t seem to reveal itself just by trial and error, as some programs do. At this point I experience that as a drawback in Photoshop. It has a load of functions but most of them are not as easily accessible as for example in PSP.
Oh well, I don’t want to be an old nag, rambling on about why one software is better than the other. You’ve helped me a lot. Thanks !!
Joost