And now all the facts which had before seemed obscure were accounted for. It had been noticed that metallic plates were often dry when grass or wood was copiously moistened. Now, we know that metals part unwillingly with their heat by radiation, and therefore the temperature of a metal plate exposed in the open air is considerably higher than that of a neighbouring piece of wood. For a similar reason, dew is more freely deposited on grass than on gravel. Glass, again, is a good radiator, so that dew is freely deposited on glass objects,—a circumstance which is very annoying to the telescopist. The remedy employed is founded on Wells’ observations—a cylinder of tin or card, called a dew-cap, is made to project beyond the glass, and thus to act as a screen, and prevent radiation. We can now also interpret the effects of a clear sky. Clouds act the part of screens, and check the emission of radiant heat from the earth. This fact has been noticed before, but misinterpreted, by Gilbert White of Selborne. “I have often observed,” he says, “that cold seems to descend from above; for when a thermometer hangs abroad on a frosty night, the intervention of a cloud shall immediately raise the mercury ten degrees, and a clear sky shall again compel it to descend to its former gauge.” Another singular mistake had been made with reference to the power which clouds possess of checking the emission of radiant heat. It had been observed that on moonlit nights the eyes are apt to suffer in a peculiar way, which has occasionally brought on temporary blindness. This had been ascribed to the moon’s influence, and the term moon-blindness had therefore been given to the affection. In reality, the moon has no more to do with this form of blindness than the stars have to do with the formation of dew. The absence of clouds from the air is the true cause of the mischief. There is no sufficient check to the radiation of heat from the eyeballs, and the consequent chill results in temporary loss of sight, and sometimes even in permanent injury. Since clouds possess this important power, it is clear that while they are present in the air there can never be a copious formation of dew, which requires, as we have seen, a considerable fall in the temperature of the air around the place of deposition. When the air is clear, however, radiation proceeds rapidly, and therefore dew is freely formed.