difficulty of coming to conclusions about the planets if now we have got to Mars we take the big telescope that has carried us there and turn it on to the Earth;

Fig. 38.—Mars Photographed with the 40-in. Lens of the Yerkes Observatory, 1909. and let us try what we can see of our Earth from that distance. I propose to arm you with the best telescope in our possession, Fig. 39. and to ask you to look with it at the Earth. Of course I must provide you artificially with the view you will get; and Fig. 39 provides two views of our Earth, differing in some respects, but both of them better views than you would have any right to expect if you were really looking from Mars, even with our very best telescope.