give the situation of the apex of the sun's way. No doubt if we knew the distance of the stars we could then deduce the rate of the solar motion from observation of the apparent displacements of the stars. Unfortunately we are so ignorant of the distances of the great majority of the stars that the process indicated is almost wholly inapplicable with any satisfactory results. It is therefore fortunate that we have a method of investigating the problem which does not require any knowledge as to the stellar distances.
In other parts of this volume we shall dwell upon the important information which the spectroscope has been made to yield with respect to the movement of celestial bodies along the line of vision. It is of the essence of this method of research that the determination of the velocity of the body is obtained quite independently of the star's distance from the observer. The indications of the spectroscope assure us that a star is moving with a velocity of, say, five miles, or ten miles, or some other pace, per second towards the observer or from the observer, and this information has no connection whatever with the remoteness of the star. On this account it becomes specially applicable to the problem we have now to consider.
Of course it will be understood that the stars are themselves endowed with absolute movements. It is therefore not always correct to assume that when the solar system and a star are lessening their distances this is to be attributed entirely or even largely to the movement of the solar system. Both the solar system and the star are moving, and the motion observed is the resultant of the two. If, therefore, we had only a few stars wherewith to control the inquiry, the spectroscope method could add
