ADDRESS. XCm 



Consequently the number of vibrations per second which reach the ear of an 

 observer situated in the former of these directions will be somewhat smaller, 

 and the number which reach an observer situated in the opposite direction 

 somewhat greater, than if the bell had been at rest. Hence to the former 

 the pitch will be somewhat lower, and to the latter somewhat higher, than 

 the natural pitch of the bell. And the same thing will happen it the ob- 

 server be in motion instead of the bell, or if both be in motion ; in fact, the 

 effect depends only on the relative motion of the observer and the bell in 

 the direction of ^ line joining the two, — in other words, on the velocity of 

 recession or approach of the observer and the bell. Tlie effect may be per- 

 ceived in standing by a railway when a train in which the steam-whistle is 

 sounding passes by at full speed, or better still, if the observer be seated in 

 a train which is simidtaneously moving in the opposite direction. 



The present state of optical science is such as to furnish us with evidence, 

 of a force which is perfectly overwhelming, that light consists of a tremor or 

 vibratory movement propagated in an elastic medium filling the planetary 

 and stellar spaces, a medium which thus fulfils for light an office similar to 

 that of air for sound. In this theory, to difference of periodic time corresponds 

 difference of refrangibility. Suppose that we were in possession of a source 

 of light capable, like the bell in the analogous case of sound, of exciting in 

 the [Ether supposed at rest vibrations of a definite period, corresponding, 

 therefore, to light of a definite refrangibility. Then, just as in the ease of 

 sound, if the source of light and the observer were receding from or approach- 

 ing to each other with a velocity which was not insensibly small compared 

 with the velocity of light, an appreciable lowering or elevation of refrangibi- 

 lity would be produced, which would be cajiable of detection by means of a 

 spectroscope of high dispersive power. 



The velocity of light is so enormous, about 185,000 miles per second, that 

 it can readily be imagined that any motion wliich we can experimentally 

 produce in a source of light is as rest in comparison. But the earth in its 

 orbit round the sun moves at the rate of about IS miles per second ; and in 

 the motions of stars approaching to or receding from our sun we might expect 

 to meet with velocities comparable with this. The orbital velocity of the 

 earth is, it is true, onlj^ about the one ten-thousandth part of the velocity of 

 light. Still the effect of such a velocity on the refrangibility of light, which 

 admits of being easily calculated, jiroves not to be so insensibly small as to 

 elude all chance of detection, provided only the observations are conducted 

 with extreme delicacy. 



Eut how shall we find in such distant objects as the stars an analogue of 

 the bell which we have assumed in the illustration drawn from sound? 

 AVhat evidence can we ever obtain, even if an examination of their light 

 should present us with rays of definite refrangibility, of tlie existence in those 

 remote bodies of ponderable matter vibrating in known periods not identical 

 with those corresponding to the refrangibilities of the definite rays which we 

 observe? The answer to this question will involve a reference, which I will 

 cndeavo\ir to make as brief as I can, to the splendid researches of Professor 

 Xirchhoff. The exact coincidence of certain dark lines in the solar spectrum 

 with bright lines in certain artificial sources of light had previously been in 

 one or two instances observed ; but it is to Kirchhoff we owe the inference 

 from an extension of Prcvost's theory of exchanges, that a glowing medium 

 which emits bi'ight light of any particular refraugibility necessarily (at that 

 temperature at least) acts as an absorbing medium, exting-uishing light of the 

 same refrangibility. In saying this it is but just to mention that in relation 



