MOST physicists are familiar with Prof. R. W. Wood's beautiful experiment1 on the anomalous dispersion of sodium vapour. In the form in which the experiment is carried out by students in this Department, the sodium pellets are distributed evenly over about five inches in the middle part of a steel tube roughly twenty inches long and of diameter one inch, with a wall thickness slightly less than one-sixteenth of an inch. The tube is fitted with a side tube connected to a pump and a manometer, its ends being sealed off by a pair of spectacle lenses of suitable focal length. The horizontal slit is situated at the focus of the lens at the incidence end of the tube, and the vertical slit of the spectrometer is placed at the focus of the lens at the emergence end of the tube. It is best to work with a horizontal slit 1–2 mm. wide. Heating is carried out by means of six jets uniformly spaced 1 in. apart and about 0·1 in. in diameter, drilled along one side of a piece of brass tubing fed by gas and closed at the other end. The heater ia arranged about 2 in. below the middle of the steel tube, and the gas supply is adjustable to give flame jets up to 3 in. high.