410 Mr. David Gill [May 29, 



enough towards an observer it would become bluer, or if away from 

 him it would become more red in colour. Only it would require a 

 change of velocity in the moving light of some thousands of miles 

 per second in order to render the difference of colour sensible to the 

 eye. The experiment is, therefore, not likely to be frequently shown 

 at this lecture table ! 



But the spectroscope enables such changes of colour to be 

 measured with extreme precision. Here on the screen is the most 

 splendid illustration of this that exists at present, viz. copies of three 

 negatives of the spectrum of a Aurigas, taken at Potsdam in October 

 and December of 1888, and in March 1889. (Fig. 2.) 



The white Hue (the picture being a positive) represents the bright 

 line Hy given by the artificial light of hydrogen, the strong black 

 line in the picture of the star spectrum corresponds to the black 

 absorption line which is due to hydrogen in the atmosphere of the 

 star. 



Why is it that the artificial hydrogen line does not correspond 

 with the stellar line in these three pictures ? The answer is, either 

 the star is moving towards or from the earth in the line of sight, or 

 the earth is moving from or towards the star. But in December the 

 earth in its motion round the sun is moving at right angles to 

 the direction of a Aurigae, why then does not the stellar hydrogen 

 line agree in position with the terrestrial hydrogen line ; the simple 

 explanation is that a Aurigee is moving with respect to the sun. 



In what way is it moving ? Well, that also is clear : the stellar 

 line is displaced towards the red end of the spectrum, that is to say 

 the star light is redder than it should be in consequence of a motion 

 of recession ; this proves that the star is moving away from us, and 

 measures of the photograph show the rate of this motion to be 15 J 

 miles per second. We also know that in October the earth in its 

 motion round the sun is moving towards a Aurigae nearly at the same 

 rate as we have just seen that a Aurigas is running away from the 

 sun. Consequently, at that time, their relative motions are nearly 

 insensible, because both are going at the same rate in the same 

 direction, and we find accordingly in October, that the positions of 

 the stellar and artificial hydrogen lines perfectly correspond. 

 Finally, in March, the earth in its motion round the sun is moving 

 away from a Aurigae, and as a Aurigae is also running away from 

 the sun the star-light becomes so much redder than normal that the 

 stellar hydrogen line is shifted completely to one side of the hydrogen 

 and artificial line. 



The accuracy of these results may be proved as follows : — 



If we measure all the photographs of a Aurigae which Dr. Vogel 

 has obtained we can derive from each a determination of the relative 

 velocity of the motion of the star with respect to our earth. 



Of course these velocities are made up of the velocity of motion 

 of a Aurigae with respect to the sun (which we may reasonably 

 assume to be a uniform velocity) and the velocity of the earth due to 



