The principle which lies at the bottom of the determination is in itself simple enough, though its application is complicated in such a manner as to render the investigation a very difficult one. A wayfarer passing up the centre of a street lighted on both sides by lamps will see that the lamps in front of him appear to open out and separate from one another as he advances, while those that he is leaving behind him have an opposite motion, appearing to close in upon one another. Now, with regard to the solar motion, if the case were absolutely simple, the same effect would be produced upon the stars among which we are moving; that is to say, were the stars absolutely fixed, and our system alone in motion among them, there would appear to be a general thinning out or retreating of the stars from the point towards which the sun is moving, and a corresponding crowding together of them towards the point, directly opposite in the heavens, from which it is receding. In actual fact the case is not by any means so simple, for the stars are not fixed; they have motions of their own, some of them enormously greater than the motion of the sun. Thus the apparent motion caused by the advance of our system is masked to a great extent by the real motion of the stars. It is plain, however, that the perspective effect of the sun's motion must really be contained in the total motion of each star, or, in other words, that each star, along with its own real motion, must have an apparent motion which is common to all, and results from our movement through space. If this common element can be disentangled from the individual element, the proper motion of each star, then the materials for the solution of the problem will be secured. It has been found possible to effect this disentanglement, and the results of all those who have attempted the problem are, all things considered, in remarkably close agreement.