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32 from A in the same line, their place among the stars being at W, will be seen from the centre of the Earth, the former at S, and the latter at Y. So, again, the bodies N and 0, which appear to an observer at A to be both situated at T among the stars, will be seen by an observer at the centre of the Earth to be the former at Q,, the latter at It. It will be perceived that the parallax or angular movement diminishes as we ap- proach the Zenith ; and the bodies M and K, which appear to an observer at A to be in the same line, and to be situated at J among the stars, will be seen by an observer at the Earth’s centre to be in the same position. It is by the measurement of the parallax of the Moon, Planets, and Sun, as seen from different points of the Earth’s surface, and by calculations founded on these, that our knowledge of their distances is obtained. The immense distance of the fixed stars, however, prevents any difference in their relative position from being perceptible to observers who survey them from different parts of the Earth’s surface at the same time. Some slight difference is seen, however, in the relative places of a few of them, during the revolution of the Earth round the Sun ; the extreme distance between the two points of observation being the diameter of the Earth’s orbit, or about 190 millions of miles. This, which is called the annual parallax, affords our only means for even guessing at the distances of the nearest" fixed stars. DIAGRAM XXYI. The purpose of this figure is to explain the fact that we do not see the Sun and other heavenly bodies precisely in their real places in the sky, in consequence to the refractive power of the atmosphere. According of the laws of Optics, a ray of light passing through a void space is in some degree bent from its course as soon as it enters the atmosphere ; and it is bent more and more as it passes from the rarer through the denser layers of the atmosphere, in its approach towards the