Page:The Moon (Pickering).djvu/91

Rh the lake itself does not appear in that sketch. The large dark areas known as seas upon Mars have their counterparts, too, in the dark regions surrounding the main crater.

Figure 3 was drawn only 0.7 day before full moon. Figure 4 was photographed 0.8 day after full, and Figure 5 was drawn only 1.7 days after it. But from full moon until lunar midday at the formation in question, or 7.4 days after sunrise, it is geometrically impossible for any shadows to be visible, and for a day or two before and after that the shadows would be too small to be recognisable. Therefore, none of the markings shown in these three figures can be due to shadow, but all must owe their origin to some surface discolouration whose intensity and shape vary with the interval during which it has been exposed to the Sun. Figure 6 was photographed 2.7 days after lunar midday, but even here it is evident that most of the dark regions are identical with those in Figure 4, and are therefore not due to shadow. In Figures 1, 2, 7 and 8, on the other hand, the presence of true shadow is plainly visible, combined with, and in some cases indistinguishable from, the surface discolouration.

With regard to the so-called double canals of Mars I may say at once that, although I have often looked for them, and sometimes looked when others told me they were visible to them, yet I have never succeeded in seeing them. Since 1894 I have had little opportxmity to examine Mars under favourable conditions, but a few years ago I was able to show, from the observations of others, that the double canals had this curious property, namely, that their linear separation was inversely proportional to the diameter of the object-glass of the telescope and directly proportional to the distance of the planet. In other words, if we use a telescope of twice the diameter we shall find the same canals will measure only half as many miles apart. Again, when Mars gets to be twice as far from the Earth as formerly, we shall find that the canal-diggers have placed their second canal twice as far from the first as it was before! The inference that I believe must be drawn from these facts is that, while the canals themselves are undoubtedly genuine, their doubling is an optical illusion, due to some peculiarity of the eye, which many astronomers are capable of seeing while many others are not. No double canals, properly so called, have been detected upon the Moon.

Although on the whole the lunar canals are much smaller and perhaps broader, in proportion to their length, than those of Mars, yet on account of the nearness of the Moon its canals are much more readily seen than the Martian ones. At the time the two