With such ideas in vogue we can well understand that the astronomers would point their lately extremely sharpened instruments toward our ruby-coloured neighbour in the sky when, in 1909, it passed very close to the earth under conditions particularly favourable to accurate observations, more so in fact than they had been during the seventeen preceding years. Numerous astro-physicists, among them the world’s foremost representatives of their discipline, have repeatedly turned their spectroscopes toward Mars in order to ascertain whether water vapour was present there or not. In the spectrum of the Sun we find several so-called “rain-bands” due to the fact that the light before it reaches the apparatus has passed through the moisture of the air. The more humid the air the more strongly developed these rain-bands are. If we direct the instrument first on the Moon, which lacks an atmosphere and therefore also moisture, and then on Mars, which for the sake of simplicity we assume standing close to the Moon-disk, a difference should appear in the spectra of these two bodies, provided moisture is present in the atmosphere of Mars. The rain-bands ought to be more pronounced in sunlight that has passed the atmosphere of Mars (passed it twice as the light is reflected by the surface of the planet) than in light reflected from the naked Moon. The bands appear of course in both spectra as the light on its final stage to the spectroscope passes the atmosphere of the Earth, which never is free from moisture. In this manner Huggins and Janssen, scientists of world-wide fame, believed that they had demonstrated the presence of water vapour on Mars. Campbell, on the other hand, the prominent director of the Lick Observatory, made similar investigations of the planet in 1894, and so did a French astronomer, Marchand, in 1896 and 1898, both under unusually favourable circumstances as the former installed his instrument 1283 m. (5200 ft.) and the latter 2860 m. (9370 ft.) above sea level, but neither found any indication of moisture in the atmosphere of Mars. It is evident that the observations would be far more accurate if we could eliminate the moisture from the atmosphere of the Earth, in which case no rain-bands would appear in the spectrum of the Moon. It would then be unnecessary to compare the two spectra; we would only have to determine whether rain-bands were present in the spectrum of Mars or not. We can never entirely avoid the water vapour of the air, but its influence may be greatly reduced by making our observations from high mountains or in desert climates where the air is comparatively dry, that is free from water vapour. Investigations undertaken where the air is dry deserve therefore more confidence than those handicapped by greater humidity. The observations by Campbell and Marchand fall in the former category, and it would, therefore, appear that the presence of water vapour on Mars to any extent worth mentioning is highly doubtful.