In this investigation, the magneto‐optic apparatus was used, but it was modified to some extent from the original apparatus used by Allison. The modifications included a mercury vapor arc and a monochromator to furnish a steady source of monochromatic light, and a photoelectric cell and amplifier circuit to read the minima. The apparatus gave a distinct minimum, at 32.9 on the electrical path scale, for all substances tested which contained vitamin A but did not show this minimum for similar substances in which this vitamin was lacking. Pure carotene did not give this minimum. Pure carotene after irradiation with ultraviolet light, however, did give it.