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16 is proportional to the refraction, we may state that its density probably does not exceed the one ten-thousandth part of our own

The question may now be asked, Is there any evidence that the Moon has any atmosphere at all? When the Moon is crescent-shaped some observers believed they detected a faint twilight prolonging the cusps or horns of the Moon. Again, in certain craters such as Stevinus, the central peaks can under favourable circumstances be seen for some hours before they are illuminated by the Sun. This latter phenomenon, however, is probably due to light reflected from some of the brilliant snow surfaces in the immediate vicinity. Perhaps the strongest evidence that we possess of a lunar atmosphere, based on direct observation, is found when the Moon occults a bright planet such as Jupiter. Under these circumstances a dark band is always seen crossing the planet, tangent to the edge or limb of the Moon. This absorption is never seen at the dark limb of the Moon, indicating thereby that the absorbing medium, whatever it is, is condensed to a solid by the intense cold that must prevail during the lunar night.

Some astronomers have attempted to explain this dark band as a mere contrast effect, due to the fact that the surface of the Moon is brighter than that of the planet; this would not be so were Venus, the planet, occulted by the Moon, but, unfortunately, favourable occultations of Venus are rare. Nevertheless, we have a very convincing proof that the dark band is a real phenomenon and not a mere subjective effect of contrast, inasmuch as it has been photographed. A photograph of an occupation of Jupiter was taken at the Harvard Observatory station at Arequipa, Peru, August 12, 1892. The two equatorial belts of the planet were well shown, while the dark absorption band due to the Moon's atmosphere was seen at right angles to them and tangent to the surface of the Moon. We thus have ocular proof that the Moon has an atmosphere. In the succeeding chapters we shall find further evidence bearing upon this subject.

The astronomers of a past generation believed that the Moon had no atmosphere at all, and it was one of the more interesting questions of the old astronomies as to what had become of it. We believe that the Moon at the present time possesses less atmosphere than it did formerly, and we shall now endeavour to explain why this is so.

When the Moon parted company from the Earth it is fair to assume that they divided their common atmosphere between them in proportion to their respective masses. Since the Moon's mass is to that of the Earth as 1 to 81.4 and its surface as 1 to 13.5, its