as they strolled back toward the lodge , the professor recounted dougs confession : how he had obtained the slide from a friend in another department who had pulled the same trick on a fellow-student ; how he had calculated from karls rehearsal the point in his script where the 'double-peaks ' would appear ; how he had palmed the slides from the projection table in the interlude , inserted the extra one , and then replaced them in the carousel just as the lights went out .