We know that sound is sent out in all directions through the medium of the air, a substance invisible and impalpable, by means of a motion that is communicated successively from one part of the air to the next; and as this movement has the same speed in all directions, it must form spherical surfaces that keep enlarging until at last they strike the ear. Now there can be no doubt that light likewise reaches us from a luminous substance through some motion caused in the matter lying in the intervening space,--for we have seen above that this cannot take place through transmission of matter from one place to another. If, moreover, light requires time for its passage--a matter we shall discuss in a moment--it will then follow that this movement is caused in the substance gradually, and therefore is transmitted, like sound, by surfaces and spherical waves. I call these _waves_ because of their likeness to those formed when one throws a pebble into water, which are examples of gradual propagation in circles, although from a different cause and on a plane surface. In regard to the question of light requiring time for its transmission, let us consider whether there is any experimental evidence against it. What experiments we can make here on the earth with sources of light placed at great distances (although indicating that it does not take a sensible time for light to pass over these distances) are subject to the objection that these distances are yet too small, and that we can only argue that the movement of light is enormously fast. M. Descartes thought it to be instantaneous and based his opinion upon much better reasons taken from the eclipse of the moon. Yet as I shall make clear, even this evidence is not decisive. I shall state the matter in a somewhat different way from his in order more easily to exhibit all the consequences. Suppose S to be the position of the sun, E A part of the orbit of the earth, S E M a straight line intersecting in M, the orbit of the moon, represented by the circle A M. Now if light requires time--say an hour--to move the distance between the earth and the moon, then [at the time of an eclipse] it follows that when the earth has come to E its shadow, or the stoppage of the light of the sun, will not yet have reached M [the moon], and will not for an hour. Counting from the instant the earth reaches E, it will be an hour before it will reach M if it is to be obscured there. This eclipse will not be seen from the earth for yet another hour. Suppose that during these two hours the earth has moved to X, the moon appearing eclipsed at M, the sun still being seen at S. For I assume as does Copernicus that the sun is fixed and since light moves in straight lines, is always seen in its true position.