xcii REPORT — 1869. 



If in respect of these phenomena optics received much aid from astro- 

 nomy, the latter science has been indebted to the former for information 

 which could not otherwise have been obtained. The motions and the masses 

 of the heavenly bodies are revealed to us more or less fully by astronomical 

 observations; but we could not thus become acquainted with the chemical 

 nature of these distant objects. Yet, by the api^lication of the spectroscope 

 to the scrutiny of the heavenly bodies, evidence has been obtained of the 

 existence therein of various elements known to us by the chemical examina- 

 tion of the materials of which our own earth is composed ; and not only 

 so, but light is thrown on the state in which matter is there existing, which, 

 in the case of nebuloe especially, led to the formation of new ideas respecting 

 their constitution, and the rectification of astronomical speculations pre- 

 viously entertained. I shall not, however, dwell further on this part of the 

 subject, which is now of some years' standing, and has been mentioned by 

 more than one of j'our former Presidents, but will pass on to newer re- 

 searches in the same direction. 



"We are accustomed to apply to the stars the e]}ithei fixed. Night after 

 night they are seen to have the same relative arrangement ; and when their 

 places are determined by careful measurement, and certain small correc- 

 tions due to known causes are applied to the immediate results of observa- 

 tion, they are found to have the same relative distances. But when instead 

 of days the observations extend over months or years, it is found that the 

 fixity is not quite absolute. Defining as fixity invariabihty of position as 

 estimated with reference to the stars as a whole, and comparing the posi- 

 tion of any individual star with those of the stars in its neighbourhood, we 

 find that some of the stars exhibit " proper motions," show, that is, a pro- 

 gressive change of angular position as seen from the earth, or rather as they 

 would be seen from the sun, which we may take for the mean annual place 

 of the earth. This indicates linear motion in a direction transverse to the 

 line joining the sun with the star. IJiit since our sun is merely a star, a 

 line drawn from the star exhibiting proper motion to our sun is, as regards 

 the former, merely a line drawn to a star taken at random, and therefore 

 there is no reason why the star's motion should be, except accidentally, in a 

 direction perpendicular to the line joining the star witli our sun. We must 

 conclude that the stars, inchiding our own sun, or some of them at least, 

 are moving in various directions in space, and that it is merely the trans- 

 versal component of the whole motion, or rather of the motion relatively 

 to our sun, that is revealed to us by a change in the star's apparent place. 



How then shall we determine Avhether any particular star is approaching 

 to or receding from our sun ? It is clear that astronomy alone is powerless 

 to aid us here, since such a motion would be unaccompanied by change of 

 angular position. Here the science of optics comes to our aid in a remark- 

 able manner. 



The pitch of a musical note depends, as we know, on the number of 

 vibrations which reach the ear in a given time, such as a second. Sup- 

 pose, now, that a body, such as a bell, which is vibrating a given num- 

 ber of times per second, is at the same time moving from the observer, the 

 air being calm. Since the successive pulses of sound travel all with the 

 velocity of sound, but diverge from different centres, namely, the successive 

 points in the bell's path at which the bell was when those pulses were first 

 excited, it is evident that the sound-waves will be somewhat more spread 

 out on the side from which the bell is moving, and more crowded together 

 on the side towards which it is moving, than if the IjcU had been at rest. 



