light and shade. It is on this account impossible to observe the moon well at the time when it is full, for then no conspicuous shadows are cast. The best time for seeing any object usually coincides with the time when the boundary between light and shade passes through the neighbourhood, and then the features are brought out with exquisite distinctness.

Plate VII.* gives an illustration of the lunar scenery, the object represented being known to astronomers by the name Triesnecker. The district included is only a very small fraction of the entire surface of the moon, yet the actual area is very considerable, embracing as it does many hundreds of square miles. We see in it various ranges of lunar mountains, while the central object in the picture is one of those remarkable lunar craters which are the characteristic features of lunar scenery. This crater is about twenty miles in diameter, and it has a lofty mountain in the centre, the peak of which is just shown, illuminated by the rising sun.

A typical view of a lunar crater is shown in Plate VIII. This is no doubt a somewhat imaginary sketch. The point of view from which the artist is supposed to have taken the picture is one quite unattainable by terrestrial astronomers, yet there can be little doubt that it is a fair representation of the objects on the moon. We should, however, recollect the scale on which it is drawn. The vast crater must be many miles across, and the mountain at its centre must be thousands of feet high. The telescope will, even at its best, only show the moon as well as we could see it with the unaided eye if it were only about 250 miles away instead of being 240,000. We must not, therefore, expect to see any details on the moon even with the finest telescopes, unless they were coarse enough to be visible at a distance of 250 miles. A view of England from a distance of 250 miles would only show London as a sort of coloured spot, by contrast with the general surface of the country.

* This sketch has been copied by permission from the very beautiful view in Messrs. Nasmyth and Carpenter's book, of which it forms Plate XI. The other illustrations of lunar scenery in Plates VII., VIII., IX., are from the same. The photographs were obtained from models carefully constructed to illustrate the features on the moon.