In the course of an investigation of the refractivity of gaseous bromine conducted with a Jamin refractometer it was observed that, as the temperature changed, very slight variations of the density of the vapour could be quickly and accurately recorded. This fact suggested that an optical method would be useful for measuring vapour pressures or densities at low pressures, especially in the case of substances which attack mercury. The following observations on the vapour of bromine serve as an illustration of the possibilities of the method, and have brought to light an interesting phenomenon, which is described below. A thin glass bulb, containing 1 or 2 c. c. of Kahlbaum’s pure bromine, was sealed to a tube communicating both with the pump and the refractometer tube. The temperature of the bromine was reduced to —80°C., at which its vapour pressure is negligible, and the bulb was completely evacuated, so as to remove all traces of hydrobromic acid which might have formed by the action of the bromine on the tap-grease since the previous experiment. The refractometer tube was separately evacuated, and the two were put in connection by opening a tap. The cooling bath was then slowly brought up to atmospheric temperature by stages, while an observer counted the interference bands which crossed the field of view of the refractometer.