"It's not a myth; it's an old wives' tale," he says, as his fingers skim over the shelf, plucking out an ancient book here and there. "There's generally at least a seed of truth in those. In this case, the casing around that seed--and, speaking of 'casing,' I think your use of the word may be St. Mungo's jargon; I'm not entirely sure what you mean by it. In this case, the casing around that seed is that the damage isn't usually noticeable until a caster's used the dark arts quite a lot. You know the exceptions."
Arms piled with books, he turns back to the reading room, speaks as he walks. "As far as I know there haven't been any recent tests; I'll rephrase. The tests done at the time were done by the best and most rigorous wizardly minds then living, and the conditions of the tests are very nearly satisfactory even by modern standards."
He puts the books down on the table, and groups them. "These," meaning the top row, "are the relevant original sources. It's not a bad idea to start with Dee for a thorough grounding, even though he doesn't address the topic in any satisfactory manner even in the Hermeticus Oscurro. Lao Wukong is more precise and convincing, but one has to have a fair understanding of the Taoist style of wizardry as well as philosophy to make much sense of his terminology, and Alonso Quixano was a very fine researcher before his own psyche deteriorated." He looks annoyed. "He left his treatise on glamours as an agent of cultural change half-finished to go chase faerie gold, but that was later. At any rate, this row you will find invaluable in making sense of the first, which is all very poetically encoded to confuse the layman, muggle, and philistine."