RENEWED attention to the “chromatic octave” tempts me to suggest an experiment. There used to be a gentleman, Smith, I believe, by name, who refuted the undulatory theory by means of a disc, divided into black and white sections, which he whirled with very high velocities, producing colours (so the Times positively stated) varying according to the velocities. It is plain that such a result might on the contrary confirm the theory, if, for instance, the disc were divided into 400 black and 400 white sectors, and whirled at the rate of one or two million million times a second. It is also plain that Mr. Smith, in the words of an authority who has been quoted in your columns for weightier judgments, would have blown his disc into smoke first. But once a second is only 40 octaves below a million million times; and it is just possible that something practicable between the two might throw light on the “chromatic octave,” among other things. There are some obvious objections: the question is, whether they make it not worth while to repeat Mr. Smith's experiments.