XII
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Whenever we wish to observe an eclipse of the sun, we place on the ground basins filled with oil or pitch. The thick liquid is not easily disturbed, and therefore retains the images it receives. Images, I may observe, cannot be seen except in a liquid at rest. Then we are in the habit of noting how the moon obstructs the sun, and by the interposition of her body hides his, which is so much larger, sometimes partially, if it so fall out that she only encounter a portion of his orb, sometimes completely. The latter is called a total eclipse: it quite shuts out the light and shows us the stars; it occurs when the centre of the two bodies lies in the same straight line. Now, just as the image of both sun and moon can be seen on earth, so it is in the case of mock suns in the atmosphere. The still air is so compact and yet clear that it can receive the sun’s likeness. Other clouds receive it, but let it go if they are either in motion, or thin, or black. The moving clouds disperse it, the rare let it slip, the black and impure do not take the impress of it, just as on earth soiled objects do not reflect an image.