While suffering some of the annoyances seemingly inseparable from re-vaccination at too advanced an age, I was led to the curious observation presently to be described. I was unable to sleep, except in “short and far between” dozes, from which I woke with a sudden start, my eyelids opening fully. I found by trial that this state of things became somewhat less intolerable when I lay on my back, with my head considerably elevated. In this position I directly faced a gas jet, burning not very brightly, placed close to a whitish wall, and surrounded by a ground glass shade, through which the flame could be prominently perceived. The portions of the wall surrounding the burner were moderately illuminated, and hyperbolic portions above and below somewhat more strongly. I observed, on waking, that the gas flame seemed for a second or two to be surrounded by a dark crimson ground, though itself apparently unchanged in colour. Gradually, after the lapse of, at the very utmost, a couple of seconds, everything resumed its normal appearance. As this phenomenon appeared not only to be worthy of observation in itself, but to furnish me with something definite to reflect upon, which is far the best alleviation of annoyances similar to those from which I was suffering, I determined to watch it, transitory as it was, feeling assured that I should have many opportunities of observing it.