When I center a small object on top of a large object, I want to have the smaller object mostly opaque, but to have just its edges fade off to being transparent, to make a smooth transition between them.

A radial gradient transparency effect blurs more than just the edges and doesn't conform to the shape of the smaller object. I think I need a mask, but haven't been able to build the right one. Can anyone point me to a tutorial on this?

(I hope you'll forgive what must be a common question. I think the reason I can't find the answer is that I don't know the right terms to look up; things like "transparent edges" and "gradient" aren't getting me there.)

I figured this out.

1. The Feather tool under Filters --> ABCs.
2. The excellent description under #6 on this post: http://inkscape-forum.andreas-s.net/topic/84362
3. I think the ideal solution would be something like what's described here http://wiki.inkscape.org/wiki/index.php/User_talk:Davidhewitt#6.29_Feather, but I can't find anything that works quite so easily.

what about the blur control in the Fill and Stroke dialog ? You can choose the amount of blur you want for you objet and it affects its edge. I don't know if my answer is relevant but...