the light path, to the end mirror, represented by D in the formula previously given, was 200 centimeters. The apparatus was in a sealed metal case filled with helium. The conclusion was that any indicated ether-drift must be less than 2.5 kilometers per second; this limiting value was later reduced by Illingworth to 1 kilometer per second.
Professor A. Piccard and E. Stahel, of Brussels, thinking that the height above the earths surface might influence the ether-drift effect, placed an interferometer in a balloon which ascended to an altitude of 2500 meters.19 The balloon was rotated about a vertical axis by means of a propeller. The interferometer had a light path in which D was 280 centimeters; it had a self-recording device and a thermostatic control; it was enclosed in a metal case which was evacuated. The indicated velocity of ether-drift might have been as large as 7 kilometers per second, which was the limit of precision. This interferometer was later taken to the summit of the Rigi in Switzerland, at an altitude of 1800 meters, where the observations showed an upper limit to the possible ether-drift of 1.5 kilometers per second.20
The late Professor Michelson, together with F. G. Pease and F. Pearson, used an interferometer mounted in the laboratory of the Mount Wilson Observatory in Pasadena, having a light path, D, equal to 1616 centimeters, which was later increased to 2592 centimeters. The readings were made in the vertical axis of the interferometer, the observer being located in the room above the apparatus. The results gave no displacement as great as one-fiftieth of that to be expected on the supposition of an effect due to a motion of the solar system of three hundred kilometers per second.21
Professor Georg Joos, working at Jena, used an interferometer mounted on a quartz base suspended in an evacuated metal housing and provided with photographic registration. The interferometer had a light path, D, equal to 2099
19 A. Piccard and E. Stahel, Comptes Rendus 183, 420
(1926); Naturwiss. 14, 935 (1926).
20 A. Piccard and E. Stahel, Comptes Rendus 185, 1198
(1927); Naturwiss. 16, 25 (1928).
21 A. A. Michelson, F. G. Pease and F. Pearson, Nature
123, 88 (1929); J. Opt. Soc. Am. 18, 181 (1929).
centimeters. The results indicated that any existing ether drift could not exceed 1 kilometer per second.22
In three of the four experiments, the interferometers have been enclosed in heavy, sealed metal housings and also have been located in basement rooms in the interior of heavy buildings and below the level of the ground; in the experiment of Piccard and Stahel, a metal vacuum chamber alone was used and in the experiment of Michelson, Pease and Pearson, the interferometer was in the constant temperature vault but did not have a vacuum case. If the question of an entrained ether is involved in the investigation, it would seem that such massive and opaque shielding is not justifiable. The experiment is designed to detect a very minute effect on the velocity of light, to be impressed upon the light through the ether itself, and it would seem to be essential that there should be the least possible obstruction between the free ether and the light path in the interferometer. It is planned to make a direct study of this factor of the problem.
In none of these other experiments have the observations been of such extent and of such continuity as to determine the exact nature of the diurnal and seasonal variations.
While the interferometer used by Kennedy is more sensitive than that of ordinary type, it is doubtful whether the precision of the result equals that obtained from the very large number of readings made under all conditions of temperature and season with the interferometer of the usual type which is much less sensitive to disturbing causes.
The limitations of the direct-reading method have been recognized but it has been adopted because of its simplicity and because it permits the accumulation of a large number of readings in the shortest time. It is believed that any lack of precision in making a single reading is fully compensated by the large number of readings and by the use of an interferometer of longer light-path and therefore of greater initial sensitivity. The interferometer used in the experiments here reported has a light-path, D, equal to 3203 centimeters.
22 G. Joos, Ann. d. Physik [5] 7, 385 (1930).
Other Evidences of Cosmic Motion
The various astronomical determinations of motion of the solar system in space, by the nature of the methods employed, indicate relative motion and do not directly give any information as to an absolute motion. However, several recent important experiments in diverse fields seem to give evidence of a cosmic motion. Dr. Esclangon, Director of the Paris Observatory, has made elaborate studies of earth tides (deformation of the earth's crust) and of ocean tides. In the latter work he considered 166,500 observations extending over a period of nineteen years.23 There are component tidal effects which indicate a motion of the solar system in the plane which contains the sidereal time meridian of 4½h and 16½h 23
By a study of the reflection of light, Esclangon finds strong evidence for what he calls an optical dissymmetry of space with its axis of symmetry in the meridian of 8 hours and 20 hours, sidereal time. This effect would be explained by an ether-drift and the results are in striking agreement with the ether-drift observations here reported.24
Many recent observations on cosmic rays show a very definite maximum of radiation in the direction indicated by the meridian of 5 hours and 17 hours, sidereal time. The very extensive observations of Kolhörster and von Salis, Büttner and Feld and of Steinke all show this effect.25 Observations made on the nonmagnetic ship Carnegie show the same effect for the observations made between 30 north and 30 south latitude.26
Evidences of galactic motions which are related more or less directly to the absolute motion of the solar system have been found by Harlow Shapley studying interstellar matter, by J. S. Plaskett from investigation of the motion of B-type stars, and by G. Strömberg from researches on star clusters and nebulae.27
23 E. Esclangon, Comptes Rendus 182, 921 (1926); 183, 116 (1926).
24 E. Esclangon, Comptes Rendus 185, 1593 (1927).
25 Kohlhörster, Steinke and Büttner, Zeits. f. Physik 50, 808 (1928).
28 Report Carnegie Inst. 27, 255 (1928).
27 Harlow Shapley, Nature 122, 482 (1928); J. S.
Plaskett, Science 71, 152 (1930); G. Strömberg, Astrophys.
J. 61, 353 (1925).
L. Courvoisier has made researches of several types to discover evidences of the absolute motion of the earth. His experiments relate to the reflection of light, the deformation of the earth, the elongations of Jupiters satellites, and to the aberration constant. R. Tomaschek and W. Schaffernicht have made observations on related subjects.28
There are several anomalies in astronomical observations of less definite character, which, however, might be explained by the existence of an ether drift. Such anomalies occur in connection with the observed constant of aberration, standard star places and clock corrections determined at different times of day.
Karl G. Jansky of the Bell Telephone Laboratories has found evidences of a peculiar hissing sound in short wave radio reception, which comes from a definite cosmic direction lying in the meridian of 18 hours sidereal time.29
Acknowledgments
The experiments here presented have involved the taking of an enormous amount of observational material, by far the greater part of which was for the purpose of making adjustments and for preliminary trials of conditions; while only the smaller portion, which is still very large, has been used in the final calculations. The reduction of this mass of material has been exceedingly laborious. No other experiment comes to mind which has involved such an amount of detail and such extended study. This has required considerable attention from many different persons. The writer is under special obligation to Professor J. J. Nassau, of the Department of Astronomy of Case School of Applied Science, for very great assistance in the analysis and in the mathematical solution of the numerical and astronomical features of the work since the beginning of the Mount Wilson observations in 1921. Dr. G. Strömberg and other members of the staff of the Mount Wilson Observatory have given advice and assistance of
28 L. Courvoisier, Astronomische Nachrichten, Nos. 5416, 5519, 5599, 5715, 5772, 5910. R. Tomaschek and W. Schaffernicht, Astronomische Nachrichten, Nos. 5844,5929; Ann. d. Physik 15, 787 (1932).
29 Karl G. Jansky, Electronics 6, 173 (1933).
