Before we leave the Moon it may be well to say a few words about its colour. Mädler states in agreement with several other observers that Mare Serenitatis, a “sea” on the Moon’s north side (25° latitude) just to the right of the centre meridian (see Fig. 27) is remarkable for its beautiful pure green colour, while Mare Crisium about 16° Lat. N. near the right edge of the Moon is of a dark grey-green hue. In Mare Humorum (about 22° Lat. S., not far from the edge of the Moon, see Fig. 28) grey and dark green shades alternate and in Mare Frigoris, just inside the lunar north pole, the colour is a dingy yellowish-green. In other words, the characteristic colour of the great lava seas is apparently green. This agrees closely with conditions on the Earth where similar formations are coloured green by silicates of ferrous protoxides, certain species of which are called green-stones. Franz, however, questions the observations of Mädler and professes the belief that very light craters appear bluish and assumes this to be a contrast effect to the general yellow hue of the Moon. Langley investigated the lunar radiation with the spectroscope and found that the ratio of blue to yellow was smaller in the moonlight than in the sunlight, for which reason the general colour of the moon resembles that of yellow sandstone. A very interesting observation was made at the Lowell observatory when investigating the spectrum of the sparse light reflected from the Earth to the portions of the Moon not exposed to the sunlight. It proved to be of a far more blue tinge than sunlight reflected from the Moon. Our conclusion must be that the Earth shines with a blue lustre. This is perfectly natural, as the diffused light which reaches us after having been scattered by particles suspended in the air (and by gas molecules as well) is a deep blue and there exists no reason why that part of the light which is thrown outward into space should be of a different colour. The Earth, therefore, is blue in contradistinction to Mars which is red, on account of its desert surface, and Venus which is bright white. The cloudy portions around the equator and the poles should appear light blue from without and should be separated by dark blue bands over the so-called horse latitudes, under which the cloudless desert regions are located on either side of the equator. (Compare the title page illustration.)