212 On some Recent Spectroscopic Researches. [April, 



peculiar to each. Now, by ordinary methods of observation, that 

 part only of a star's motion which is transverse to the Hne of sight 

 can be detected, for the motion a star might have in the visual 

 direction, either directly towards the earth or from it, would not 

 cause any apparent change of position of the star, and would there- 

 fore remain undetected. 



The method of photometry was clearly too coarse and too un- 

 certain. Too coarse, for the eye, even when aided by suitable 

 instruments, could not hope to distinguish the minute increase or 

 decrease of brightness which would correspond to any velocity we 

 could with probability assign to the stars, even if the observations 

 were repeated at intervals of a thousand years. Too uncertain, 

 for if the variations in the transparency of our atmosphere could be 

 certainly eliminated, the large number of stars, of which the light 

 is known to be variable, would forbid us to regard any alterations 

 of brightness that might be observed, as trustworthy indications of 

 the approach or recession of the star. 



Now this radial motion of a star, which eluded our methods of 

 observation, does record itself most fortunately in small alterations, 

 which can be distinguished in the spectrum of its light. If the 

 star be approaching the earth, a line, either light or dark, in its 

 spectrum will be found to have moved from its proper place towards 

 the blue end of the spectrum ; if the star be receding fi-om the earth, 

 the line wull have moved in the oj)posite du'ection, towards the red. 

 The amount of shift of position produced by a star's motion will 

 express exactly the proportion which its motion bears to the velocity 

 of light. As the rate of propagation of hght is known, the velocity 

 of the star may be found. 



The refrangibility, or the colour of a ray of light, or what is 

 the same thing, the place in the spectrum which the light would 

 take after it had passed through a prism, is determined by the 

 number of pulsations or waves which meet the eye, or fall upon 

 the prism, in a second of time. The special character which dis- 

 tinguishes red light from violet light consists in that the waves 

 of red light are nearly as long again as those of extreme violet 

 light. Now the velocity of propagation through the ether is 

 precisely the same for all the colours of the spectrum, lied, yellow, 

 green, blue light, emitted by a distant star, would reach the earth 

 at the same instant, and it is for this reason that a new star, at the 

 first moment that its light falls upon a human eye, appears of its 

 true colour. Light travels with a velocity of about 185,000 mih s 

 in a second of time, a series of waves therefore extending through 

 185,000 miles enters the eyes each second. Now as it is upon the 

 length of the waves, or upon the number contained in the series 

 that enters the eye in a second, that a judgment is formed of the 

 colour of the light, or the place of the light in the spectrum after 



