By the way, I also saw a reference in Naomi Rosenblum's "A World History of Photography" to a direct postive "cyanotype" process. Now I have read before about the difference between blueprints (negative/positive process) and bluelines (direct positive needing development in ammonia fumes). See this Photo.Net thread: and especially the remark by Kelly Flanigan.

Is there someone who has more info or links on Hippolyte's direct positive cyanotype process, and especially the chemistry behind it?