On either theory it is difficult to account for the bright rays around such craters as Tycho and Copernicus, for they run in nearly straight lines over craters, cones and seas alike, often for hundreds of miles; and at no phase angle do they cast shadows. They can therefore not be elevations above the surrounding surface. Some regard them as cracks filled up later with matter of a light tint, as quartz fills rock veins in our own state.
I have stated the objections to the theories rather than the arguments in their favor because the objections must in some way be removed before either theory can be accepted as satisfactory. Some recent work by Professor R. W. Wood, however, may be referred to here which to some extent seems to favor the theory of origin by volcanic or similar explosive forces. He has photographed the Moon in light of different wave lengths, first in yellow light, then in violet and finally in ultraviolet light, and the three sets of photographs show some marked differences in appearance. For example, a large dark patch just above the crater Aristarchus appears on the ultraviolet picture, which is practically invisible in the yellow one and only faintly visible in the violet one. Professor Wood took two specimens of volcanic tufa of about the same color, one of which photographed light and the other dark in rays of ultraviolet light. Placing a small chip from the dark specimen upon the light one he secured effects exactly reproducing those shown by the Aristarchus spot. Analysis then showed that the dark chip contained iron and traces of sulphur. Experimental photographs of many rock specimens having iron stains failed to give these effects, but by taking the specimen of tufa which had photographed light in the ultraviolet picture and forming on a spot on its center a very thin deposit of sulphur—so thin as to be invisible to the eye—he obtained photographs showing the spot quite black in the ultraviolet, gray in the violet and invisible in the yellow. This makes it appear probable that there is a deposit of sulphur near Aristarchus on the Moon. More extended work along this line is needed, however, before any theory of crater formation can be based upon it.
I have said that the Moon is a world where nothing ever happens. Some astronomers would take exceptions to this, and it is perhaps well to remind ourselves that a universal affirmative (or negative) is a dangerous form of statement. It is quite conceivable, for instance, that a large meteorite might strike the Moon at some time
