The _parallax_ of the fixed stars had been long a subject of inquiry. If a body describe a circle, and a spectator on that body be unconscious of his own motion, all other bodies will appear to describe circles parallel to that of the spectator’s motion, and, absolutely speaking, equal to it; consequently, the greater the distance of the body from the spectator, the smaller will its apparent annual motion be; and it will not be circular, because the projection of the circle upon the apparent sphere of the heavens will foreshorten, and cause it to appear oval. If we suppose a star to describe an oval in the course of a year, the consequence will be that it will pass the spectator’s meridian sometimes before a star in the centre of the oval, sometimes after it; sometimes nearer to the pole of the heavens, and sometimes more distant; and the nature of the motion of this kind which would arise from parallax can be mathematically deduced. If the star be so distant that the oval is too small to be detected by measurement (which is hitherto the case with the fixed stars), then no alteration of place will be perceived on this account; but if an oval large enough to be observed be described in the course of a year, then the test of the phenomenon arising from the earth’s motion in its orbit is as follows:—Imagine a plane always passing through the centre of the sun, the centre of the earth, and the centre of the oval described by the star, then the place of the star in its oval must be in that plane; or draw the shortest distance on the globe from the centre of the oval to the sun, and the star will be on the point of the oval which lies in that distance.