Let us now see how the brightness is produced that sometimes envelops 1 the heavenly bodies. History has put on record that, on the day of the late Emperor Augustus’ entrance into Rome on his return from Apollonia, a parti-coloured circle, such as is wont to be seen in a rainbow, appeared round the sun. The Greeks call this a _Halo_; our most appropriate name for it is a Crown. Let me explain how it is formed. When a stone is thrown into a pond, the water is observed to part 2 in numerous circles, which, very narrow at first, gradually widen out more and more until the impulse disappears, lost in the surface of the smooth water beyond. Let us suppose something of the same kind to occur in the atmosphere. When condensed it is capable of receiving an impact: the light of sun, moon, or any heavenly body encountering it forces it to recede in the form of circles. Moisture, be it observed, and air, and everything else that takes shape from a blow, is driven into the same form as that possessed by the object that strikes it. Now every kind of light is round. Therefore, the air when struck by light will 3 assume this form. Accordingly the Greeks gave the name Threshing-floor (_i.e._ Halo) to a brightness of this kind, because spaces set apart for threshing corn were, as a rule, round. Be the better name threshing-floors, or be it crowns, there is no reason to suppose that they are formed in the neighbourhood of the heavenly bodies. They are a very long distance from them, though as seen from the earth they seem to touch and encircle them. In reality such an image is formed not very far from the earth, but the wonted 4 frailty of human vision is deceptive, and we imagine the ring is formed close round the heavenly body itself. But no such thing could possibly occur in the neighbourhood of the sun and stars, as there is nothing but thin ether there. It is only when bodies have become rough and dense that shape can be impressed upon them. In subtle bodies there is no point on which form can lay hold or to which it can adhere. A phenomenon of the same nature as the halo may often be witnessed in baths, because the atmosphere is thick and dark: it is most frequent when the wind is in the south, when the air is heaviest and most dense.