After a lecture on animal heat towards the end of last session, in which I described Langley's original form of bolometer, with its two iron gratings, and mentioned that Dr Stewart, of Philadelphia, had referred to the use of tinfoil and leadfoil gratings for physiological purposes in his text-book, I started to make a bolometer for our department. I should mention that some time ago Professor M'Kendrick, impressed by the probability of the bolometer being of use to physiologists, had communicated with Professor Langley, who kindly sent him, from Baltimore, literature dealing with the subject, and stated that the instrument was not made by an instrument-maker.