Some devotees of Richard Wagner have suggested, not wholly in jest, that the best way to enjoy the master’s operas is with one’s eyes closed. For not only have few stage presentations of his monumentally conceived and famously lengthy music dramas ever approached the cosmic sublimity of their underlying compositions, but nowadays it appears mandatory for them to be produced as bizarrely as possible, and despite the composer’s detailed visual instructions to the contrary. All of which has made the history of the composer’s original conceptions—primary evidence of which is on view in the Morgan Library & Museum’s “Wagner’s Ring: Forging an Epic”—feel like something of a rediscovery.