But forasmuch as she is not only enlightened herself, but also sends back hither the image of her illumination, this confirms us yet further in what we say touching her substance. For reflections and reverberations are not made upon any thing which is rare, and of thin and subtile parts; nor is it easily to be imagined how light can rebound from light, or one fire from another. But that which is to make the reverberation or reflection must be solid and firm, that a blow may be given against it and a rebounding made from it. As a proof of this, we see that the air transmits the sun, and gives him a way to pierce quite through it, not obstructing or driving back his rays; but on the contrary from wood, stones, or clothes put in the sun, there are made many reflections of light and many illuminations round about. So we see that the earth is illuminated by him, not to the very bottom, as the water, nor thoroughly and all over, as the air, through which the beams of the sun have a clear passage; but just such a circle as surrounds the moon surrounds also the earth; and as much of the earth as this circle includes, so much does the sun enlighten, the rest being left without light; for what is illuminated both in the one and in the other is little more than an hemisphere. Permit me therefore now to conclude after the manner of geometricians by proportions. If there are three things which the light of the sun approaches, the air, the moon, and the earth, and if we see that the moon is enlightened by him, not as the air, but as the earth, it is of necessity that those two things must have one and the same nature, which of one and the same cause suffer the same effects. 19. Now when all the company began highly to commend Lucius’s harangue; This is excellently well done of you, Lucius (said I to him), that you have to so fine a discourse added as fine a proportion, for you must not be defrauded of that which is your due.