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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



one look at a spectrum of fair length, or even at 

 a correctly-tinted painting of the solar spectrum, 

 and note how utterly unrecognizable to ordinary 

 vision is the difference of tint for even the twen- 

 tieth part of the distance between medium green 

 and medium yellow on one side, or medium blue 

 on the other, and he will recognize how utterly 

 hopeless it would be to attempt to appreciate the 

 change of color due to the approach or recession 

 of a luminous body shining with pure green light 

 and moving at the tremendous rate of 100 miles 

 per second. It would be hopeless, even though 

 we had the medium green color and the changed 

 color, either toward yellow or toward blue, placed 

 side by side for comparison — how much more 

 when the changed color would have to be com- 

 pared with the observer's recollection of the me- 

 dium color, as seen on some other occasion ! 



But this is the least important of the difficul- 

 ties affecting the application of this method by 

 noting change of color, as Doppler originally pro- 

 posed. Another difficulty, which seems some- 

 how to have wholly escaped Doppler's attention, 

 renders the color-test altogether unavailable. We 

 do not get pure light from any of the celestial 

 bodies except certain gaseous clouds of nebula?. 

 From every sun We get, as from our own sun, all 

 the colors of the rainbow. There may be an ex- 

 cess of some colors and a deficiency of others in 

 any star, so as to give the star a tint, or even a 

 very decided color. But even a blood-red star, 

 or a deep blue or violet star, does not shine with 

 pure red light, for the spectroscope shows that 

 the star has other colors than those producing 

 the prevailing tint, and it is only the great excess 

 of red rays (all kinds of red, too) or of blue rays 

 (of all kinds), and so on, which makes the star 

 appear red, or blue, and so on, to the eye. By 

 far the greater number of stars or suns show all 

 the colors of the rainbow nearly equally distrib- 

 uted, as in the case of our own sun. Now, imag- 

 ine for a moment a white sun, which had been 

 at rest, to begin suddenly to approach us so rap- 

 idly (traveling more than 10,000 miles per sec- 

 ond) that the red rays became orange, the orange 

 became yellow, the yellow green, the green blue, 

 the blue indigo, the indigo violet, and that the 

 violet waves became too short to affect the sense 

 of sight. Then, if that were all, that sun, being 

 deprived of the red part of its light, would shine 

 with a slightly-bluish tinge, owing to the relative 

 superabundance of rays from the violet end of* 

 the spectrum. We should be able to recognize 

 such a change, yet not nearly so distinctly as if 

 that sun had been shining with a pure green light, 



and, suddenly beginning to approach us at the 

 enormous rate just mentioned, changed in color 

 to full blue. Though, if that sun were all the 

 time approaching us at the enormous rate im- 

 agined, we should be quite unable to tell whether 

 its slightly-bluish tinge were due to such mo- 

 tion of approach or to some inherent blueness in 

 the light emitted by the star. Similarly, if a 

 white sun suddenly began to recede so rapidly 

 that its violet rays were turned to indigo, indigo 

 to blue, and so on, the orange rays turning to 

 red, and the red rays disappearing altogether, 

 then, if that were all, its light would become 

 slightly reddish, owing to the relative super- 

 abundance of light from the red end of the spec- 

 trum ; and we might distinguish the change, yet 

 not so readily as if a sun shining with pure green 

 light began to recede at the same enormous rate, 

 and so shone with pure yellow light. Though, if 

 that sun were all the time receding at that enor- 

 mous rate, we should be quite unable to tell 

 whether its slightly-reddish hue were due to such 

 motion of recession or to some inherent redness 

 in its own lustre. But in neither case would that 

 be all. In the former, the red rays would indeed 

 become orange ; but the rays beyond the red, 

 which produce no effect upon vision, would be 

 converted into red rays, and fill up the part of the 

 spectrum deserted by the rays originally red. 

 In the latter, the violet rays would indeed be- 

 come indigo ; but the rays beyond the violet, or- 

 dinarily producing no effect, would be converted 

 into violet rays, and fill up the part of the spec- 

 trum deserted by the rays originally violet. Thus, 

 despite the enormous velocity of approach in one 

 case and of recession in the other, there would 

 be no change whatever in the color of the sun in 

 either case. All the colors of the rainbow would 

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

 therefore, still be a white sun. 



Dopplcr's method would thus fail utterly, even 

 though the stars were traveling hither and thith- 

 er with motions a hundred times greater than the 

 greatest known stellar motions. 



This objection to Doppler's theory, as origi- 

 nally proposed, was considered by me in an arti- 

 cle on " Colored Suns " in Eraser's Magazine for 

 January, 18G8. His theory, indeed, was origi- 

 nally promulgated, not as affording a means of 

 measuring stellar motions, but as a way of ac- 

 counting for the colors of double stars. It was 

 thus presented by Prof. Nichol, in a chapter of 

 his " Architecture of the Heavens," on this spe- 

 cial subject : 



" The rapid motion of light reaches indeed one 



