Page:Popular Science Monthly Volume 71.djvu/195

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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 autobiography and memoirs of the late Duke of Argyle, edited by his wife, have lately been published in two large volumes. Perhaps most men of science, on being asked offhand for an estimate of the duke, would reply that he was an amiable dilettante, with more enthusiasm than knowledge. In a wav, this is correct enough; but given