The representation of the moon in pictures of the ordinary size (some three feet long by two in height) is a case in which the artist habitually--one may almost say invariably--departs greatly from scientific truth, and it is a question as to whether he is justified in what he does. Take first the case of the low-lying moon near the horizon as contrasted with the high moon. Everyone knows that the moon (and the sun[4] also) appears to be much bigger when it is low than when it is high. Everyone who has not looked into the matter closely is prepared to maintain that the luminous disc in the sky--whether of moon or of sun--not merely seems to, but actually does, occupy a bigger space when it is low down near the horizon than when it is high up, more nearly overhead. Of course, no one nowadays imagines that the moon or the sun swells as it sinks or diminishes in volume as it rises. Those who think about it at all, say that the greater length of atmosphere through which one sees the low sun or moon, as compared with the high, magnifies the disc as a lens might do. This, however, is not the case. If we take a photograph of the moon when low and another with the same instrument and the same focus when it is high, we find that the celestial disc produces on the plate (as it does on our eyes) a picture-disc of practically the same size in both positions. In fact, the high moon or sun produces a picture-disc of a little larger size than the low moon or sun. I have here reproduced (Pl. IV) a photograph, published by M. Flammarion, in which the moon has been allowed to print itself on a photographic plate exposed during the time the moon was rising, and it is seen that the track of the moon has not diminished in width as it rose higher and higher. No one will readily believe this, yet it is a demonstrable fact. Astronomers have made accurate measurements which show that there is no diminution of the disc under these circumstances, but a slight increase--since the moon is a very little nearer to us when overhead than when we see it across the horizon. [Illustration: Plate IV.--The track of the rising moon registered by continuous exposure of a photographic plate. It is given here in order to show that the diameter of the visible disc of the moon does not diminish as it rises. The slight increase in the breadth of the track registered by the moon's disc is probably due to a little distortion caused by the side portion of the lens. After M. Flammarion. The actual width of the moon's disc as printed here is a little over one eighth of an inch, which, if we regard it as "a picture" and not merely as a mechanical record, implies that the observer's eye is only about 14-1/2 inches distant from the picture plane instead of the more usual 18 inches, which corresponds to a diameter of the pictured moon's disc of between 1/6th and 1/7th of an inch (.156 inch).]