In the next place, let me carry you with me one step more. Here is a glass bulb from which the greater part of the air has been removed. We say, therefore, that there is a _vacuum_ in the bulb. It is impossible for us to remove absolutely every trace of air from the bulb, and so produce what would be called a _perfect vacuum_; but we can imagine it to be accomplished, and we can picture to ourselves the glass bulb absolutely deprived of every trace of air or other material substance. The question then arises—Is the bulb really empty, or is there still _something_ in its interior? The same inquiry may be put in another way. The air we breathe forms an atmosphere which surrounds our earth as a garment, but it decreases rapidly in density as we ascend. At a height of about 50 miles above the earth there is reason to believe the air is exceedingly rarefied and, except for the presence of meteoric dust, the space between the sun and the earth, and between the stars and the earth, is in all probability a highly perfect vacuum, in the sense that it is empty of generally diffused matter. The question then arises—Is interstellar space absolutely and completely empty? We know perfectly well that rays of light come to us from the sun and stars through this empty space, and a fact of capital importance is, that these rays of light, swift-footed though they are, take time to travel. It was long ago suspected that this was the case, and the celebrated Galileo made the first experimental attempt to determine the velocity of light. No real knowledge on the subject was gained, however, until after he had made his discovery that the planet Jupiter is accompanied by four moons (a fifth moon has been discovered since), and that these rotate round the planet in definite periods of time, constituting, therefore, the “hands” of a perfect celestial clock. The sunlight, falling on the great globe which forms the body of the planet Jupiter, casts behind it a conical shadow; and the little moons, in their rotation, are plunged into this shadow cone at intervals, and then for a time become invisible, or eclipsed.