Time.

Books Inevitably Archive.org has a wonderful printing of Gaston Leroux's The Phantom of the Opera in an English translation, published by Grossett and Dunlap in 1911. The opening of the prologue is remarkable:

"THE Opera ghost really existed. He was not, as was long believed, a creature of the imagination of the artists, the superstition of the managers, or a product of the absurd and impressionable brains of the young ladies of the ballet, their mothers, the box-keepers, the cloak-room attendants or the concierge. Yes, he existed in flesh and blood, although he assumed the complete appearance of a real phantom; that is to say, of a spectral shade."

As ever one of the best elements of their scans is that it has the whole book including covers and the library administration. The book was heavily used in its time. Perhaps it was a core text at the Brigham Young University where it originated.