This psychic process, by which unconscious memories become conscious in dreams, is of considerable interest and importance because it lends itself to many delusions. Not only the ignorant and uncultured, but even well-trained and acute minds, are often so unskilled in mental analysis that they are quite unable to pierce beneath the phenomenon of conscious ignorance to the deeper fact of unconscious memory; they are completely baffled, or else they resort to the wildest hypotheses. This is illustrated by the following narrative received twelve years ago from a medical correspondent in Baltimore. 'Several years ago,' he writes, 'a friend made a social call at my house and in the course of conversation spoke very enthusiastically of Mascagni's _Cavalleria Rusticana_, the first performance of which in the United States he had attended a few nights previously. I had never even heard of the opera before, but that night I dreamed that I heard it performed. The dream was a very vivid one, so vivid that several times during the next day I found myself humming airs from the dream opera. Several evenings later I went to the theatre to see a comedy, and before the curtain rose the orchestra played a selection which I instantly recognised as part of my dream opera. I exclaimed to a lady who was with me: "That selection is from _Cavalleria Rusticana_." On inquiring of the leader of the orchestra such proved to be the case.' Now, at that period, shortly after the first appearance of _Cavalleria Rusticana_, portions of it had become extremely popular and were heard everywhere, by no means merely on the operatic stage. It was difficult not to have heard something of it. There cannot be the slightest doubt that my correspondent had heard not only the name but the music, though, writing at an interval of some years, he probably exaggerated the extent of his unconscious recollections. This seems the simple explanation of what to my correspondent was an inexplicable mystery. Other people, like the late Frederick Greenwood, not content to remain baffled, go further and regard such dreams as 'dreams of revelation,' as they also consider that class of dreams in which the dreamer works out the solution of a difficulty which he had vainly grappled with when awake.