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mention of individual specimens, brief but interesting chapters upon the different kinds of precious stones included; we note one on the agate, another on the cutting, polishing and staining of agates at Oberstein, and others similar.

III. MISCELLANEOUS SCIENTIFIC INTELLIGENCE.

1. Report of the Committee appointed by the Smithsonian Institution to Award the Hogdkins Fund Prizes.-The Committee of Award for the Hodgkins prizes of the Smithsonian Institution has completed its examination of the two hundred and eighteen papers submitted in competition by contestants.

The committee is composed of the following members: Doctor S. P. Langley, chairman, ex-officio, Doctor G. Brown Goode, appointed by the Secretary of the Smithsonian Institution, Assistant Surgeon-General John S. Billings, by the President of the National Academy of Science, Professor M. W. Harrington, by the President of the American Association for the Advancement of Science. The Foreign Advisory Committee, as first constituted, was represented by Monsieur J. Janssen, Professor T. H. Huxley, and Professor von Helmholtz; and after the recent loss of the latter, Doctor W. von Bezold was added. After consultation with these eminent men, the committee decided as follows: First prize, of ten thousand dollars, for a treatise embodying some new and important discoveries in regard to the nature or properties of atmospheric air, to Lord Rayleigh, of London, and Professor William Ramsay, of the University College, London, for the discovery of argon, a new element of the atmosphere.

The second prize, of two thousand dollars, is not awarded, owing to the failure of any contestant to comply strictly with the terms of the offer.

The third prize, of one thousand dollars, to Doctor Henry de Varigny, of Paris, for the best popular treatise upon atmospheric air, its properties and relationships. Doctor de Varigny's essay is entitled "L'Air et la Vie."

(Signed), S. P. Langley, G. Brown Goode, John S. Billings, M. W. Harrington. August 9, 1895.

Supplementary Report of the Committee appointed by the Smithsonian Institution to Award the Hodgkins Fund Prizes.After having performed the function to which the committee was called, as announced by the circular of the secretary of the Smithsonian Institution, dated March 31, 1893, which function did not include the award of any medals, there remained several papers to which the committee had been unable to give any prize and to which they had felt desirous to give. some honorable mention, and on their representing this to the Smithsonian Institution they have been commissioned to do so, and also to give certain medals of silver and bronze which had been subsequently placed at their disposition.

The committee has decided that honorable mention should be made of the papers, twenty-one in number, included in the following list, which also gives the full names, titles, and addresses

of the authors, and the mottoes or pseudonyms which in four instances were employed. To three of the papers a silver medal is awarded and to six a bronze medal:

Honorable Mention with Silver Medal.

PROF. A. L. HERRERA and DOCTOR VERGARA LOPE, of the City of Mexico: "La Atmosfera de las altitudes y el bienstar del hombre."

MR. C. L. MADSDEN-"Geo "-Helsigör, near Copenhagen, Denmark: Thermographical Studies.

MR. F. A. R. RUSSELL, of London, Vice-President of the Royal Meteorological Society of Great Britain: "The Atmosphere in Relation to Human Life and Health."

Honorable Mention with Bronze Medal.

M. E. DEBURAUX-DEX and M. MAURICE DIBOS-" Spes "-of Rouen, France: "Etudes des courents aériens continentaux et de leur utilization par des ærostats long-courriers."

DOCTOR O. JESSE, of Berlin: "Die leuchtenden Nachtwolken."

DOCTOR A. LOEWY, of Berlin: "Untersuchungen über die Respiration und Cirkulation unter verdünnter und verdichteter Sauerstoffarmer und sauerstoffreicher Luft."

MR. ALEXANDER MCADIE—“ Dalgetty "—of Washington: "The known properties of atmospheric air considered in their relationships to research in every department of natural science, and the importance of a study of the atmosphere considered in view of these relationships; the proper direction of future research in connection with the imperfections of atmospheric air and the conditions of that knowledge with other sciences."

MR. HIRAM S. MAXIM, of Kent, England: "Natural and Artificial Flight." DOCTOR FRANZ OPPENHEIMER and DOCTOR CARL OPPENHEIMER "E pur si muove"-of Berlin. Germany: Ueber atmosphärische Luft, ihre Eigenschaften und ihren Zusammenhang mit dem menschlichen Leben."

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Honorable Mention.

MR. E. C. C. DALY, of University College, London: "The decomposition of the two constituents of the atmosphere by means of the passage of the electric spark."

PROFESSOR F. H. BIGELOW, of Washington: "Solar and Terrestial Magnetism and their Relation to Meteorology."

DOCTOR J. B. COHEN, of Yorkshire College, Leeds, England: "The Air of Towns."

DOCTOR F. J. B. CORDEIRO, U. S. N., of Washington: " Hypsometry." PROFESSOR EMILE DUCLAUX, of the French Institute, Paris, France: "Sur l'actinométrie atmosphérique et sur la constitution actinique de l'atmosphère." PROFESSOR DOCTOR GIESELER, of Bonn, Germany: "Mittlere Tagestemperaturen von Bonn, 1848-88."

DOCTOR LUDWIG ILOSVAY VON NAGY ILOSVA, Professor in the Royal Joseph Polytechnic School, Budapest, Hungary: "Ueber den unmittelbar oxydirenden Bestandtheil der Luft."

DOCTOR A. MAGELASEN, of Christiania, Norway: "Ueber den Zusammenhang und die Verwandschaft der biologischen, meteorlogischen und kosmischen Erscheinungen."

DOCTOR A. MARCUSE, of the Royal Observatory, Berlin, Germany: "Die atmosphärische Luft."

PROFESSOR C. NEES, of the Polytechnic School, Copenhagen, Denmark: "The use of kites and chained air-balloons for observing the velocity of winds, etc." SURGEON CHARLES SMART, U. S. A.. Washington: "An Essay on the Properties, Constitution and Impurities of Atmospheric Air, in Relation to the Promo. tion of Health and Longevity."

DOCTOR F. VIAULT, of the Faculty of Medicine, Bordeaux, France: "Décou verte d'une nouvelle et importante propriété physiologique de l'Air atmosphérique (action hématogène de l'air raréfié).”

(Signed) S. P. Langley, G. Brown Goode, John S. Billings, M. W. Harrington. August 9, 1895.

THE

AMERICAN JOURNAL OF SCIENCE

[THIRD SERIES.]

ART. XXIX.-Recent Progress in Optics;* by W. LECONTE STEVENS.

Introductory.

THE reviewer who aspires to give an account of recent progress in any department of science is met at the outset by two causes for embarrassment: What beginning shall be selected for developments called recent? What developments shall be selected for discussion from the mass of investigations to which his attention has been called? So rapidly is the army of workers increasing, and so numerous are the journals in which their work is recorded, that the effort to keep up with even half of them is hopeless; or, to borrow a simile employed by the late Professor Huxley, "We are in the case of Tarpeia, who opened the gates of the Roman citadel to the Sabines, and was crushed under the weight of the reward bestowed upon her."

I have selected a single branch of physics, but one which can scarcely be treated rigorously as single. From the physical standpoint optics includes those phenomena which are presented by ether vibrations within such narrow limits of wave length as can affect the sense of sight. But these waves can scarcely be studied except in connection with those of shorter and of longer period. Whatever may be the instruments employed, the last one of the series through which information is carried to the brain is the eye. The physicist may fall into error by faulty use of his mathematics; but faulty use of the senses is a danger at least equally frequent. Physiological * Address delivered by the Vice-President of Section B at the meeting of the American Association for the Advancement of Science, August 29.

AM. JOUR. SCI-THIRD SERIES, VOL. L, No. 298-OCTOBER, 1895.

optics has of late become transferred in large measure to the domain of the psychologist; but he in turn has adopted many of the instruments as well as the methods of the physicist. The two cannot afford to part company. If I feel particularly friendly to the psychologist, more so than can be accounted for by devotion to pure physics, it may be fair to plead the influence of old association. If I am known at all in the scientific world, the introduction was accomplished through the medium of physiological optics. But, with the limitations imposed, it is not possible even to do justice to all who have done good work in optics. If prominence is assigned to the work of Americans, it is not necessary to emphasize that this Association is made up of Americans; but, with full recognition of the greater spread of devotion to pure science in Europe, of the extreme utilitarian spirit that causes the value of nearly every piece of work in America to be measured in dollars, we are still able to present work that has challenged the admiration of Europe, that has brought European medals to American hands, that has been done with absolute disregard of monetary standards; work that has been recognized, even more in Europe than in America, as producing definite and important additions to the sum of human knowledge.

In drawing attention to some of this work it will be a pleasant duty to recognize also some that has been done beyond the Atlantic; to remember that science is cosmopolitan. The starting point is necessarily arbitrary, for an investigation may last many years and yet be incomplete. To note recent progress it may be important to recall 'what is no longer recent.

Light Waves as Standards of Length.

You are, therefore, invited to recall the subject of an address to which we listened in this section at the Cleveland meeting in 1888, when Michelson presented his "Plea for Light Waves. In this he described the interferential comparer, an instrument developed from the refractometer of Jamin and Mascart, and discussed various problems which seemed capable of solution by its use. In conjunction with Morley, he had already used it in an inquiry as to the relative motion of the earth and the luminiferous ether, and these two physicists together worked out an elaborate series of preliminary experiments with a view to the standardizing of a metric unit of length in terms of the wave length of sodium light. By use of a Rowland diffraction grating Bell had determined the sodium wave. length with an error estimated to be not in excess of one part in two hundred thousand. Could this degree of accuracy be

*

*This Journal, May, 1886, p. 377.
Ibid., December, 1887, p. 427.
Ibid., March, 1887, p. 167.

*

surpassed? If so, it must be not so much by increased care in measurement as by increase of delicacy in the means employed. The principle applied in the use of the interferential comparer is simple enough; the mode of application cannot be clearly indicated without a diagram, but probably all physicists have seen this diagram, for it was first brought out eight years ago." By interference of beams of light, reflected and transmitted by a plate of plane-parallel optical glass, and then reflected back by two mirrors appropriately placed, fringes are caught in an observing telescope. One of the mirrors is movable in front of a micrometer screw, whose motion causes these fringes to move across the telescopic field. If the light be absolutely homogeneous, the determination consists in the measurement of the distance through which the movable mirror is pushed parallel to itself and the counting of the number of fringes which pass a given point in the field of view. According to the theory of interference, the difference of path between the distances from one face of the plate to the two mirrors should be small; beyond a certain limit interference phenomena vanish, and this limit is smaller in proportion as the light is more complex. In the case of approximately homogeneous light there are periodic variations of distinctness in the fringes. For example, assume sodium light, which in the spectroscope is manifested as a pair of yellow lines near together. In the refractometer there are two sets of interference fringes, one due to each of the two slightly different wave lengths. When the difference of path is very small, or nearly the same for both of these radiation systems, the fringes coincide. The wave length for one is about one-thousandth less than that for the other. If the difference of path is about five hundred waves, the maximum of brightness for one system falls on a minimum of brightness for the other, and the fringes become faint. They become again bright when the difference of path reaches a thousand wave lengths. The case is entirely similar to the familiar production of beats by a pair of slightly mistuned forks.

The method of interference thus furnishes through optical beats a means of detecting radiation differences too minute for resolution by ordinary spectroscopic methods. Spectrum lines are found to be double or multiple when all other means of resolving them fail; and the difficulty of attaining truly homogeneous light is far greater than was a few years ago supposed. By the new method it becomes possible to map out the relative intensities of the components of a multiple line, their distance apart, and even the variations of intensity within what

* Ibid., December, 1887, p. 427.

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