Popular Science Monthly/Volume 71/August 1907/The Progress of Science
DOES THE SPEED OF LIGHT IN SPACE DEPEND UPON ITS WAVE-LENGTH?
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 High School of Philadelphia, has solved one part of this problem. He has shown that the ultra-violet waves and the waves of the visible spectrum travel with the same velocity. For this the Franklin Institute has awarded to him one thousand dollars of the accumulated fund. There has been no lack of applications for the premium, but no portion of it has ever before been awarded. The investigating committee, consisting of Mr. Hugo Bilgram, mechanical engineer; Professor A. W. Goodspeed, of the University of Pennsylvania, and Dr. G. F. Stradling, of the Northeast Manual Training High School of Philadelphia, were unanimous in their favorable opinion.
The star Algol, or Persei, is a spectroscopic binary, that is, a study of its light shows that part of the time the star is approaching the earth and part of the time receding from it. Moreover, every 69 hours it grows less bright, only to regain its rank as a star of the second magnitude after the lapse of about 7 hours. The simplest explanation of these erratic performances is that there are two bodies, one luminous, the other opaque, revolving around their common center of mass. The dimming of brightness occurs when the opaque body gets between the earth and the luminous body. Their diameters, orbital velocities and masses have been calculated and also the distance their centers are apart.
The remoteness of Algol—it takes light 30 years to come thence to the earth—as well as its change of brightness caused it to be selected by Dr. Heyl for his investigation. In brief his method was this. He obtained records of the change of brightness of the star by photographing it at intervals in ultra-violet light produced by a transparent diffraction grating. The variation as judged by the eye was already known. If the ultra-violet waves travel faster than those belonging to the visible spectrum there would be a shifting of the time of least brightness of the image. A comparison of the two cycles of change however shows that there can not be a greater difference between the speed of the ultra-violet light and that of the visible spectrum of more than one part in 250,000.
There seems to have been no previous determination of the speed of ultraviolet waves in a vacuum. Dr. Heyl's result, in substance that the two kinds of waves do not differ in speed by more than 1 km. per second, is of high value. To be sure it has been assumed for a long time that no such difference existed, but an experimental proof is a very different thing from mere extrapolation.
The work was conducted with the 8-inch equatorial of the Central High School and extended over a period of two years. The times when the variation of Algol occurred at a suitable time of day and under appropriate conditions of the sky were rare.
As yet there seems to be no experimental demonstration that the infrared rays and those of the visible spectrum travel in space with the same speed. As far back as 1842 Wrede believed he had shown that the two speeds were different, but his work was subject to error. The method of Dr. Heyl does not lend itself to the settlement of this second part of the problem, since the infra-red rays have little effect upon a photographic plate. Let us hope that some physicist may devise an appropriate method and thus remove this gap in our knowledge of the velocity of radiation—incidentally obtaining another portion of the Boyden premium.
THE DUKE OF ARGYLE
without qualification, it does him a great injustice. He was, throughout his life, an earnest, sincere and industrious man, much interested in the advancement of his fellows and the cultivation of his own mind. Inheriting an enormous estate, and taking a most prominent part in the politics of his time, he bore on his shoulders as great a burden as a man might care to lift, without taking time and energy for scientific work. It is impossible to say what he might have done, had he devoted himself mainly to some single branch of science or literature, but one may readily believe that it would in no
wise have equalled his actual achievement as a versatile man of affairs. He was not a genius, in the ordinary acceptation of the term; but he was one of those thoroughly useful citizens who serve to hold together the diverse elements of human society. In this sense, he was a duke in fact as well as in name, and an aristocracy so typified is not without a certain justification even from our democratic point of view.
Many naturalists are familiar—and some no less tired than familiar—with the duke's controversial writings on semi-metaphysical questions relating to evolution. Fewer, we imagine, know how enthusiastically he watched the birds and other living things on his estate, and how graphically and accurately he could describe them. The following, taken from a letter to Lord Litford, should endear him to every ornithologist: "Anent the dipper, I need not say how I agree with you in loving them. I have three salmon streams in my estates which they haunt. I never allow one to be shot. We have many pairs, but they never seem to increase much. As to their propensities, I have had ocular demonstration that they eat fish, and that greedily. Twice I have seen a dipper with a fish in his bill—one was a trout or salmon fry, the other was a small flounder. This was in the seaport of the river Aray below my house. The flounder was, of course, a small one, but it was as broad as the white waistcoat of its devourer. I had a good glass, and saw the dipper emerge with the little flounder in his bill. He then took it to a large boulder stone near the bank, and began beating it to death against the stone. Twice it slipped off into the stream, and each time it was firmly pursued and brought back to the block! All aquatic piscivorous birds seem to have a way of doubling and folding up the flat fishes they catch so as to get them down, but I did not see the feat performed in the present case."
The following good story is told in another part of the same letter: "I bought two 'civette' (small owls) in Rome, and took them in a cage with me home. We travelled with Gladstone. He was immensely captivated by the brilliant yellow eyes of the birds. They fastened them on Gladstone's brown eyes with a fixed stare, and he took it into his head to try if he could stare them out of countenence. He continued to joke all the way from Rome to near Perugia, and at last the owls gave it up and looked away. He seemed as delighted as if he had won a great Parliamentary triumph." This is dated 1896. His first letter on birds, so far as the biography shows, was written in 1837—and the interest did not flag in the long interval.
We record with regret the deaths of Professor Alfred Newton, F.R.S., who held the chair of zoology and comparative anatomy at Cambridge: of Dr. Edward John Routh. F.R.S. . the mathematician, of the University of Cambridge; of Dr. Maxwell Tylden Masters. F.R.S., the English botanist and horticulturist; of Dr. Alexander Stewart Herschel. F.R.S., honorary professor of physics at the Durham College of Science; of Sir Dietrich Brandis, F.R.S., inspector general of the forests of India; of Professor Kuno Fischer, professor of philosophy at Heidelberg; of Henry G. Hanks, at one time state geologist of California, and of Mrs. Elizabeth Cabot Cary Agassiz. who in 1850 married Louis Agassiz, with whose work she was intimately associated, and whose life she wrote.
The council of the British Association for the Advancement of Science has nominated Mr. Francis Darwin, F.R.S. . foreign secretary to the Royal Society, author of important papers on physiological botany and of the 'Life and Letters of Charles Darwin,' to be president of the meeting next year, when, for the fourth time, the association will assemble in Dublin.—M. de Lapparent, professor of mineralogy and geology at Paris, has been elected permanent secretary of the Paris Academy of Sciences in succession to the late M. Berthelot.—On the occasion of the celebration of the bicentenary of the birth of Linnæus, the Linnean gold medal of the Royal Swedish Academy was awarded to Sir Joseph Hooker.—A portrait of President Eliot by Mr. John P. Sargent has been unveiled in the Harvard Union.
Dr. E. H. Sellards, for three years geologist and zoologist to the Florida University, has been appointed state geologist of Florida by Governor Broward.—Dr. E. A. Ruddiman, professor of materia medica and pharmacy at Vanderbilt University, Nashville, has been appointed chief food and drug inspector of the Department of Agriculture.—Dr. Frederick L. Dunlap, instructor in the University of Michigan, has been appointed associate chemist in the Bureau of Chemistry, and-will be a member of the board of food and drug inspection.
An Italian Association for the Advancement of Science, proposed at Milan last year, has now taken form. The first meeting will be held at Parma in September next, when it is hoped that the sister associations of Europe and America will send delegates.
Mrs. Russell Sage has given the sum of $300,000 to found what will be known as the Russell Sage Institute of Pathology as an adjunct to the City Hospital on Blackwell's Island.—Dr. Lawrence F. Flick, director of the Phipps Institute, Philadelphia, and chairman of the committee on the International Congress of Tuberculosis, which is to be held in Washington in the fall of 1908, announces that he has received $35,000 in subscriptions to a fund of $100,000 which he is raising to meet the necessary expenses.