The motion of a star may be resolved into two components, one representing its movement at right angles, and the other as parallel to the eye of the observer, The former is determined by the telescope and the latter by the spectroscope. If the star moves towards us, its vibrations producing any colour increase, and hence "the dark lines of the spectrum are pushed beyond their usual place towards the violet end; if going away from us, it is the contrary, and the dark lines are pushed towards the red "end of the spectrum. The amount of displacement in the spectrum enables us to calculate the velocity of the star according to the well known laws of optics. Thus Ajdabaran Betelguex and Rigel are actually receding from our solar system at the average rate of about 20 miles per second, while Spica Virginis and Altair are coming towards the sun at about 10 miles a second.
Text read from the printed page and cleaned; old type leaves the occasional error.