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 of the shift due to motion. If the movement of the star be towards us, then the whole system of lines is shifted towards the blue end, whereas it moves towards the red end when the star is hastening from us. The amount of the shift is a measure of the speed of the movement. This is the consideration which brings the process within the compass of practical astronomy. We need not here discuss the appliances, optical, mechanical, and photographic, by which an unexpected degree of precision has been given to the measurements. It seems that in the skilful hands of Vogel and Keeler it is possible in favourable cases to obtain determinations of the velocities of objects in the line of sight with a degree of precision which leaves no greater margin for doubt than about five per cent, of the total amount. It is truly astounding that such a degree of accuracy should be attainable under conditions of such difficulty. It must also be remembered that the distance of the object is here immaterial, unless in so far as the reduction in the brilliancy of the star owing to its distance involves a difficulty in making the observations.

As the first illustration of the extraordinary results that are now being obtained by the application of the new process, I take the case of the celebrated variable Algol. This star is a well-known object to all star-gazers; it lies in the constellation of Perseus, and its vagaries attracted notice in early times. In ages when the stars were worshipped as divinities it is not unreasonable to suppose that a star whose light varied in any extraordinary manner should naturally be viewed with some degree of suspicion as contrasted with stars that dispense their beams with uniformity. It was doubtless a feeling of