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Prix Bordin: rapport sur le concours de l'année 1867
H. Fizeau
Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences 66, 932-934 (1868).

``The prize will be awarded to the scientist who will have performed or proposed an experiment that will definitively resolve the question already studied many times previously concerning the direction of vibrations of the ether in polarized rays. "

The proposed question has given rise to an interesting study, referred to here as manuscript no 1, with this epigraph: sine experientia nihil sufficienter sciri potest.

Without yet having brought a definitive answer to the controversial problem concerning the true direction of vibration of the ether in polarized light, the author proposes a new experiment which seems suitable to resolve the question; this experiment, it is true, has not yet been performed and would certainly present considerable difficulties in its execution; nevertheless your commissioners have found it not only very ingenious and well conceived, but also based on reasoning plausible enough to merit, if not the prize, at least a reward suitable to encourage the author in the realization of his thoughts.

It relates in fact to a question that is of the highest degree of interest to our knowledge of light. We are well in agreement to accept that luminous rays consist of vibrations, that these vibrations are transversal, which is to say normal to the direction of propagation; but once the light is divided into two beams polarized a right angles, we can no longer agree if the vibrations of each polarized beam takes place in the same plane as the polarization or in a perpendicular plane.

The principle of the experiment imagined by the author and described in his Mémoire following a chapter devoted to the history of the question, is the following: if we interfere two rays that meet at 90 degrees, and we polarize one and the other in the same way, either parallel to the plane containing the two rays, or normal to this plane, we can expect that the interference will be influenced in a decisive manner by the real direction of vibration of the polarized rays.

If in fact, according to the opinion of Fresnel, this direction is normal to the plane of polarization, it follows (in considering here only the case where the two rays will be polarized in the plane which is common to both) that the vibrations of one ray and the other are normal to this plane and parallel to each other, and that, consequently, at the point of crossing, there will be strengthening or extinction of the light, according to the relative phases of the two rays, exactly as is ordinary experiments, where the interfering rays follow the same direction or cross under very small angles.

If on the other hand, according to the opinion of Neumann and MacCullach, the vibrations take place in the plane of polarization, as they are always normal to the direction of the rays, the necessary result is that, in the case considered, the direction of vibration of one and the other rays will make between them, at the point where they meet, an angle of 90 degrees. Therefore, in these circumstances, we know that, whatever the difference in phase between the two rays, it should no longer produce either strengthening, or extinction of light, and that the phenomenon of interference is replaced by changes of polarization linear, circular or elliptical, without a change in the intensity.

However the author does not limit himself to describing the principle of the experiment, he has studied in more detail, with a profound knowledge of the most delicate phenomena of optics, the experimental arrangement which seemed to him best to overcome numerous difficulties that could be expected.

First, the little known influence of non transversal vibrations must intervene in the observations, that the author hopes to overcome using photographic traces of the luminous phenomenon. Next, extremely fine fringes must be produced in these circumstances and could only be perceived with the aide of a microscope. Finally, the faintness of the luminous intensity, even using sunlight, must arise due to the very small dimension we must give the source of light destined to serve as the common origin of the interfering rays.

There exists in fact for the majority of interference phenomena, such as Young's fringes, those from Fresnel's mirrors and those which according to Arago give rise to stellar scintillation, a remarkable and necessary relationship between the dimension of the fringes and that of the luminous source, such that extremely fine fringes can only be brought into existence when the source of light has but angular dimensions that are almost undetectable; therefore, to remark in passing, it is perhaps possible to hope that in relying on this principle and in forming for example, by the means of two wide and widely-separated slits, interference fringes at the focus of large telescopes, it would become possible to obtain new data on the angular diameters of stars.

In summary, your Commissioners were unanimous in recognizing the distinguished merit of Mémoire no 1, and considering that terms of the program relate either to an experiment performed, or to an experiment only proposed, they would not have hesitated to award the prize to this work, were it not for the discussion that they have already put forth that there yet exists some uncertainty of the effectiveness of the means very skillfully devised by the author to assure the success of his experiment.

In consequence, your Commissioners have charged me to submit to the Academy the following two proposals:

1. The Contest for the Bordin prize of 1867 (question relating to the directions of vibrations of the ether) is declared ended;

2. A medal worth two thousand francs is awarded to the author of the Mémoire listed as no 1.

These proposals are adopted by the Academy.

Last Updated 2 December 1998