IN NATURE, February 22, p. 565, reference is made to an interesting observation by Prof. Luther, of the Dusseldorf Observatory. The note states that he saw the half of the disc of Mars nearest the moon become green just before occultation on December 4, 1911, and he suggests that this may have been due to a lunar atmosphere. The time was 16h. 40m. (Greenwich mean time), and I notice that the moon was full at 14h. 52m. on December 5, so that, at the time of the observation, the unilluminated crescent of the moon towards the planet must have been extremely narrow, so that the illuminated part of the lunar disc must have been quite close to the planet.