68 NebulcB and Nebular Hypotheses. [Sess. 



Moreover, we cannot expect that they are all at present at 

 the same temperature, for we cannot assume that they were 

 contemporaneous in origin ; and even if we could, the rate of 

 progress must have been indefinitely varied in individual cases. 

 Some stars are much brighter than others. This is no doubt 

 due in certain instances to the fact that the fainter are more 

 remote ; but in other cases it is due to a difference in the size 

 and mass of the stars. Now a small cinder falling from the 

 fire cools much more rapidly than one larger ; and by similar 

 reasoning we should anticipate that small stars would run 

 their luminous course more quickly than large stars would. 

 Some stars must therefore be at a later stage of development 

 than others. 



Then again, the most casual observer must have noticed 

 that the stars vary in colour. Some are white, some are 

 yellow, some are patently red. If we judge these by our 

 standards of white - heat and red - heat, we should say that 

 white stars are at a higher temperature than red stars, and 

 we should expect them to become redder as time goes on and 

 they become colder. 



So far this is largely speculation, but as it happens we have 

 proof positive that the speculation has a real foundation in 

 fact. !For by means of an instrument known as the spectro- 

 scope we can determine not only the chemical elements of 

 which a star is composed, but also the temperature at which 

 those elements exist at the star's surface. 



It would take me completely out of my way were I to 

 attempt any adequate description of this instrument now. It 

 must sufi&ce to say that the phenomenon of light is due to 

 the impact of ether waves upon our eyes. These waves are 

 of varying lengths, and the length of the wave determines 

 the colour of the light. Very short waves excite the im- 

 pression of violet or deep blue light ; waves somewhat longer 

 excite the impression of green light ; waves longer still, of 

 yellow light; and the longest waves of all excite the im- 

 pression of red light. If waves of all different lengths enter 

 the eye at the same time, the sensation of vjMte light is 

 produced. To prove that white light consists of waves of 

 different lengths, we pass the beam of light through a slit, 

 and then interpose in front of the slit an ordinary glass 



