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Extrait d'une lettre de M. Stéphan à M. Fizeau
E. Stéphan
Comptes Rendus Hebdomadaires des Séances de l'Académie des Sciences 76, 1008-1010 (1873).

Translated by Peter R. Lawson.

Astronomy - On interference fringes observed with large telescopes directed on Sirius and on numerous other stars; results which may be obtained, relating to the angular diameters of these stars. Extract of a letter from Mr. Stephan to Mr. Fizeau.

During the course of a Report on the Bordin prize, included in volume 66 of the Proceedings, you expressed yourself as follows:

There exists 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.

With this you point out an entirely new approach, that no one, to my knowledge, has yet attempted, and which nonetheless may lead to results by a path completely different from the normal methods of astronomy.

With respect to the diameter of stars, we know that the image of a star contains a bright center, surrounded by a diffraction ring. The apparent diameter of this spot, as seen from the optical center of the objective, diminishes when we increase the diameter of the objective, but it is never zero, and, if the star itself has a very small diameter, the width of the bright central spot will be increased, all around, by an amount that cannot be deduced by straightforward examination. This would no longer be the case if we could produce, within the image, contrasts of light and darkness that are a function of the diameter of the source. The use of interference phenomena therefore provides us with precisely the means to achieve this end.

Imagine one covers the objective of a refracting telescope with a screen that contains two parallel slits, A and B, symmetrically placed about the optical center of the objective, and let me suppose, for now, that they are reduced to two infinitely thin lines. We know that, if we project onto the screen parallel rays originating at the same source, we obtain Young's fringes at the focus, and that the angle subtended by the first two dark fringes as seen from the optical axis of the objective, is expressed, in seconds of arc, by the simple formula

where l represents the separation of the slits A and B, measured in millimeters; that is to say, and this is the most important point, that the angle x is inversely proportional to the separation of the two slits, no matter what telescope used.

If we sight on a star whose diameter is infinitely small, fringes will always arise, and, to make them visible, one simply needs to use a sufficiently high magnification. However, if the star has an appreciable diameter, that is to say if it projects onto the screen beams of rays in slightly different directions, for each direction there corresponds a set of fringes; these various sets overlap one on the other, and, to make the fringes disappear altogether, it suffices that the diameter of the star be equal to the angle x.

This phenomenon is yet produced when instead of two narrow slits, we introduce in the screen openings of an appreciable size.

We find ourselves therefore in the possession of a method of measurement whose sensitivity increases with the separation of the slits, that is to say with the aperture of the refractor; however, if fringes were not produced, an experiment would only be considered convincing if, the experimental setup remaining the same, certain stars produced fringes, while others, seen under precisely similar conditions, did not.

The telescope at Marseille, because of its large aperture, lends itself better to this type of study than any other instrument in France: I have recently used it, and, although my work has only begun, I hasten to relate to you my first results.

It is obvious that one must first consider the brightest stars.

On the first evening, I began exploring these, with the help of an ordinary refracting telescope furnished with a screen pierced with two narrow slits, parallel and separated by 15 centimeters. All the stars examined, even Sirius, produced very distinct fringes; although, those of Sirius were less clear than the others.

The next day, I resumed the same investigation with the [larger] telescope covered with a screen pierced by two oval shapes, that were located at opposite ends of one diameter, and whose centers were separated by about 50 centimeters. This time Sirius no longer produced fringes for me, no matter what magnification was used, although all the other stars produced fringes that were more or less distinct. Sirius was low in the sky; but the stars of Orion, of an elevation hardly greater than Sirius, presented stripes that were very clear. It should be noted moreover that, in these circumstances, the phenomenon of fringes only becomes visible if one uses a very strong magnification; I went up to a magnification of about one thousand.

Since that time, poor weather has not permitted me to resume the experiment. I am therefore very far from presenting these results as conclusive; however, in the manner that the fringes persisted, irrespective of fluctuations in the images, I am very strongly inclined to think that the disappearance of the fringes with Sirius was not wholely due to the influence of the atmosphere. I have the firm hope that later experiments will clearly demonstrate that the diameter of this star is not undetectable, and will permit an approximate assessment.

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Last Updated 4 December 1998