118 



THE NUMBER OE THE STAKS. 



rest, and if the source is receding from us, we receive fewer waves in 

 a giveij time. The sensation of shrilhiess is greater or less accord- 

 ing to the greater or less rapidity with which the waves reach our 

 ear. That is analogous to the behaviour of light. The eye is able 

 to discriminate between the rays of light which come to us with 

 different numbers of waves each second, and it discriminates between 

 them by means of the colour sense. The spectroscope enables us to 

 learn of what colours are the various rays of light which go to form 

 a given beam, and by means of it we are able to measure the number 

 of waves reaching us per second in the case of the different com- 

 ponent waves. Certain stars, however, send us light, of which the 

 number of waves reaching us per second varies from time to time, 

 and this has been interpreted in the same way as the analogous 

 phenomena of sound, as showing that the source of light is alternately 

 approaching and receding — probably (certainly in some cases) due to 

 the revolution of one star round another just as the earth revolves 

 round the sun. As to the centre of the stellar universe, no one 

 knows exactly where that is, because we do not know the bounds of 

 the universe at all correctly. The centre, like the North Pole or the 

 Equator of the earth, is probably not marked by any definite object, 

 but it is generally considered that our solar system is near the centre 

 of the universe. One reason for this conclusion is that the Milky 

 Way, which appears to be a great band of stars encircling the universe, 

 is seen by us nearly as a great circle in the sky, and is of approxi- 

 mately equal thickness in its different parts, so that we are apparently 

 near the centre of the galaxy, and therefore, according to our ideas 

 of the universe, we must be also near the centre of the latter. 



A question was asked about dark stars. In one sense most of the 

 stars of which I have been speaking, are dark stars ; tha.t is to say, 

 we never see them with the naked eye. But there are also stars 

 which we have never been able to photograph, which are known only 

 from their effect upon others. With regard to the stars being thought 

 to be suns, it was about the middle of the last century, or perhaps a 

 little earlier than that, that the distances of some stars were first 

 measured. It then became, for the first time, possible to calculate 

 from their distance, the brightness that they would have if they 

 were as near us as the sun is, and consequently how they compared 

 with the sun as to the actual amount of light which they radiated 

 The Meeting adjourned at 6.20. 



