90 PLEASANT WA YS IN SCIENCE. 



of recession in the other, there would be no change whatevei 

 in the colour of the sun in either case. All the colours of 

 the rainbow would still be present in the sun's light, and it 

 would therefore still be a white sun. 



Doppler's method would thus fail utterly, even though 

 the stars were travelling hither and thither with motions 

 a hundred times greater than the greatest known stellar 

 motions. 



This objection to Doppler's theory, as originally proposed, 

 was considered by me in an article on " Coloured Suns " in 

 Fraser's Magazine for January, 1868. His theory, indeed, 

 was originally promulgated not as affording a means of 

 measuring stellar motions, but as a way of accounting for 

 the colours of double stars. It was thus presented by 

 Professor Nichol, in a chapter of his " Architecture of the 

 Heavens," on this special subject: "The rapid motion of 

 light reaches indeed one of those numbers which reason 

 owns, while imagination ceases to comprehend them ; but it 

 is also true that the swiftness with which certain individuals 

 of the double stars sweep past their perihelias, or rather 

 their periasters, is amazing ; and in this matter of colours, it 

 must be recollected that the question solely regards the 

 difference between the velocities of the waves constituent of 

 colours, at those different stellar positions. Still it is a bold 

 even a magnificent idea ; and if it can be reconciled with 

 the permanent colours of the multitude of stars surrounding 

 us stars which too are moving in great orbits with immense 

 velocities it may be hailed almost as a positive discovery. 

 It must obtain confirmation, or otherwise, so soon as we can 

 compare with certainty the observed colorific changes of 

 separate systems with the known fluctuations of their orbital 

 motions." 



That was written a quarter of a century ago, when spec- 

 troscopic analysis, as we now know it, had no existence. 

 Accordingly, while the fatal objection to Doppler's original 

 theory is overlooked on the one hand, the means of applying 

 the principle underlying the theory, in a much more exact 



