What are the characters on this seal? The middle two seem to be 皇帝, but I can't make out the other four.
edited, according to info provided by droooze.
the recognised character in sequence:
character ５：後 (u+5f8c)
character ６：裔 (u+88d4)
character ３：皇 (u+7687)
character ４：帝 (u+5e1d)
character １, the left component is 水 (u+6c34), the left one, wait :)
edited. with help from droooze, this one should be 漢 (u+6f22)
character ２, this one would be 亙 (u+4e99)
so, it would be read as "漢亙皇帝後裔", roughly "the descendent of emperor '漢亙帝'"
have fun :)