The lunar feature which comes next in importance to the difference of tint between the so-called 'seas' and the higher lands is the existence of remarkable series of radiating streaks extending from certain important craters--centres probably of past disturbance. It is impossible to contemplate the disc of the full moon, as seen with a powerful telescope, without feeling that these systems of rays must have resulted from the operation of forces of the most stupendous nature, though as yet their true meaning is hid from us. They would be marvellous phenomena, even if they were not so mysterious--marvellous in their enormous extension, their singular brightness, and their manner of traversing 'seas,' craters, and mountain-ranges indifferently. But their chief marvel resides in the mysterious manner of their appearance as the moon approaches her full illumination. Other lunar features are most clearly recognised when the moon is not full, for then the shadows which afford our only means of estimating the height of lunar irregularities are clearly seen along the border between the bright and dark parts of her face, and we have only to wait until this border passes over any object we wish to study to obtain satisfactory evidence of its nature. It is quite otherwise with the rays. The regions occupied by these radiating streaks are neither raised nor depressed in such sort as either to throw shadows or to lie in shadow when surrounding regions are in sunlight. But when the moon approaches her full illumination, the radiating regions come into view, as bright streaks--bright even on the light-tinted lunar uplands. A mighty system of rays can be seen extending from the great crater Tycho in all directions. Other systems, scarcely less wonderful, extend from the battlemented crater, Copernicus, the brilliant Aristarchus, and the solitary Kepler. One ray from Tycho can be traced round nearly an entire hemisphere of the moon's surface. It is specially noteworthy of this great ray that, where it crosses the lunar Sea of Serenity, that great plain seems to be divided as by a sort of ridge line, the slope of the plain from either side of the ray's track being clearly discernible when the moon is near her first quarter.