J. J. THOMSON1 discovered the triatomic molecule of hydrogen by the positive ray parabolic method. It was later confirmed by Dempster2, Duane and Wendt3, and several others. Its occurrence is very frequently noted in discharge tubes, but the optimum conditions are not well defined. While investigating the Doppler effect in hydrogen positive rays, using a Wien type of discharge tube, maximum Doppler displacements corresponding to H3+ molecules in the discharge space were observed. The accompanying graph gives the maximum Doppler displacements recorded in the HI´ line of the Balmer series. A Steinheil three-prism glass spectrograph was used at two different dispersions, namely, 7.6 A. and 27.2 A. per mm. in the HI´ region.