When a beam of light comes through a prism of glass or a raindrop it is dispersed into a band of vivid colors, each denoting a particular wave-length. Though all these wave-lengths travel together in the air they part company in the glass or the water because there they no longer possess the same speed. The long waves, which produce the sensation of red, travel faster than the short, or violet waves.
Whether all wave-lengths really do travel with the same speed in air has not always been a matter of a single opinion. Lorenz and Ketteler both have found that the index of refraction for air differs by some seven parts in a million according to which end of the spectrum is employed. This means a proportionate difference in the speed of light in air for the long and for the short waves. More than a quarter of a century ago Young and Forbes, using Fizeau's method, seemed to find that the speed of the blue waves in air was 1.8 per cent, greater than that of the red ones. This result was threshed over by Lord Rayleigh, who pointed out serious objections to accepting their results. When Michelson was determining the speed of light, he paid especial attention to this question. When white light and red light were compared not the slightest trace of difference in their speeds could be detected. We may, therefore, rest assured that all waves of the visible spectrum travel with practically the same speed in air.
Now how is it in a vacuum, especially in that vast vacuum, interstellar space? If we begin by limiting our observations to our own solar system, it has been noticed that when one of its satellites goes behind Jupiter its color is just the same as when it emerges. Suppose that Young and Forbes were right and that the blue rays do travel faster than the red rays. Then when the satellite is behind the planet so that it can send no more light to the earth, the train of waves which it emitted before its eclipse, still pursues its journey toward us. If the blue waves outrun the red waves, it will be the latter which give us our last glimpse of the satellite. At disappearance it should then appear red. Similarly upon emergence the blue should be the first waves to reach the eye, but no such difference of color upon eclipse and emergence is seen. Hence we may conclude that all waves of the visible spectrum travel in space with the same speed. It is, however, well to bear in mind that the universe is larger than the solar system and that the visible spectrum by no means includes all known radiation.
In 1859 Uriah A. Borden deposited with the Franklin Institute of Philadelphia one thousand dollars to be awarded as a premium to "any resident of North America who shall determine by experiment whether all rays of light, and other physical rays, are or are not transmitted with the same velocity." This problem was restated by the board of managers thus: "Whether or not all rays in the spectrum known at the time the offer was made, namely, ^March 23, 1850. and comprised between the lowest frequency known thermal rays in the infra-red, and the highest frequency known rays in the ultra-violet... travel through free space with the same velocity."
Dr. Paul R. Heyl, of the Central