The Periscope is an optical device used by the Eds in "In Like Ed" to spy on the kids at Jimmy's birthday party in order to find out what they were planning as they thought the kids, mostly Kevin, were conducting some mysterious affair. It had a viewfinder so its user could aim it on his observation object. After a few uses by Eddy, Ed replaced the periscope with himself, discarding the actual periscope.
In Ed, Edd n Eddy's Big Picture Show, a similar periscope was used by Jonny to spy on the kids because he wanted revenge on them for beating him up as Captain Melonhead. It was camouflaged under a flowery carpet behind a fence and extended by using a long pipe so it could be used to look at the street while The Gourd was in the Melon Cave. The viewfinder of this periscope appears to be the same as the original one.