April 2, 1885] 



NA TURE 



5i7 



of the methods and task of ethnology, by " Achelis" ; a map of 

 the Congo, with accompanying description, by Herr Richard 

 Kiepert ; and a note on the additions and changes made in the 

 Chinese administrative organisation of the Thienshan region. 

 The Verhandlungen (Band xii. Xo. 2) of the same Society con- 

 tains a criticism by Herr Erman, who has for some years had 

 charge of the historical and geographical departments of the 

 Royal Library at Berlin, of the methods in which the work of 

 compiling a bibliography of the geographical works relating to 

 Germany — a " Bibliotheca geographica Germanise" — is being 

 carried out. 



At the meeting of the Geographical Society of Paris on 

 March 20, a letter was read from the French Consul at Asuncion 

 in Paraguay, giving details of the expedition sent by the Argen- 

 tine Government to explore the Pilcomayo, and to ascend to 

 the Bolivian frontier if possible. It has been found that, owing 

 to impassable rapids, the river cannot be utilised as a route 

 between Paraguay and Bolivia. The only practicable route is 

 that by land, the possibility of which was recognised in 1883 by 

 M. Thouar's expedition. M. de Cailland described the Pescadore 

 archipelago in the Formosan channel. The islands have excellent 

 roadsteads, and form the key to Formosa. M. Simonin read a 

 note on the Indian population of the United States ; and M. 

 Jules Gamier described his project of an aerial railway for 

 Pari*. 



A NEW ARRANGEMENT OF THE APPARA- 

 TUS OF THE ROTATING MIRROR FOR 

 MEASURING THE VELOCITY OF LIGHT 1 



IJAVING now been engaged for a number of years in 

 measuring the velocity of light by means of the rotating 

 mirror, I have succeeded in rearranging the apparatus in such a 

 manner as, by means simply of two mirrors, one fixed, the 

 other movable, placed at a distance of a few metres from each 

 other, to obtain, even with a very moderate velocity of rotation, 

 a deviation of the image of a fixed object as large as may be 

 desired in theory and limited in practice only by the intensity of 

 the light and the perfection of the optical apparatus. 



To describe in a few words the plan of I,. Foucault's cele- 

 brated experiment :— The rays issuing from a narrow aperture 

 fall, at a distance of I m., on a rotating mirror 14 mm. in dia- 

 meter, and, on being reflected there, traverse an object-glass 

 placed as near the mirror as possible. This object-glass throws 

 an image of the aperture on a spherical concave mirror having a 

 radius of 4 m. placed at a distance of 4 m. from the rotating 

 mirror. A second mirror, in all respects perfectly correspond- 

 ing with the first, receives the reflected pencil, which produces 

 a fixed image of the rotating mirror, and transmits a movable 

 image of the apertme to a third mirror, and so on. Foucault's 

 apparatus comprised five similar mirrors. The last, in which a 

 fixed image was formed, reflected on the fourth the light, which 

 its previous course and so came back to the rotating minor, 

 which again in turn transmitted it deviated in respect of its rota- 

 tion by an angle twice as large as that at which it had turned 

 when performing the double passage of the mirrors, i.e. twice 

 20 m. The velocity of rotation being 400 revolutions per 

 second, Foucault obtained a deviation of 7 mm. 



One of the objections taken to Foucault's experiment and the 

 values he deduced from it respecting the velocity of light, is the 

 •f that deviation. It is known how he ingeniously 

 cleared the difficulty by substituting for the measurement of the 

 deviation that of the distance of the aperture from the rotating 

 mirror producing a determined deviation. He did not, how- 

 ever, disguise the fact that the advantage of this substitution is 

 mire specious than real, and he brought forward the 

 plan of an apparatus compo-ed of a series of objectives and of a 

 concave mirror, by means of which the passage of the light 

 extended to several hundreds of metres. He had even 

 at the Observatory, the place where his new experi- 

 ments might be carried out.' 



I have to confess that, in endeavouring to take advantage of 

 Foucault's scheme, whether by means of object-glass or of mir- 

 ror-, I --.ruck on such difficulties as caused me to desist from the 

 prosecution of my researches by either of the methods indicated. 



In the United States in 1879 Mr. Michelson put in operation 

 the experiment of the rotating mirror at great di-tances, but 

 under an arrangement which brings the experiment much nearer 



' Paper, by M. C. Wolf, in the Comptts RcnJus for February 9. 



to the celebrated one of MM. Fizeau and Breguet than that of 

 Foucault. The aperture from which the light issues was placed 

 at a distance of about 30 English feet (Q'I5 m.) from the rotating 

 mirror, the diameter of which amounted to \\ inches (3 "2 cm.). 

 A simple non-achromatic lens, 7 inches ('7'8 cm.) in diameter, 

 and having a focus of 150 feet (45 75 m. ) was placed in such a 

 manner as to throw an image of the aperture, seen by reflection 

 in the rotating mirror, on the surface of a plain mirror, 7 inches 

 in diameter, placed normally to the line passing through the 

 centres of the two mirrors and the lens, at a distance of 1 986 '23 

 feet (605 80 m.) from the rotating mirror. The pencil then 

 returns on itself, and gives an image of the aperture coinciding 

 with it, point for point, when the mirror is fixed, deviated as 

 soon as it rotates. The lineal displacement of the image during 

 a rotation of 258 revolutions per second amounted to Il4'l5mm. 

 The advantage, however, of such a large displacement seems to 

 be counterbalanced by the inferior quality of the image. A lens 

 7 inches in diameter, and with a focus of 150 feet will, even 

 under the best conditions, necessarily give an image bounded by 

 very large fringes of diffraction, which atmospheric agitations 

 transform into a luminous blot so ill-defined that, as Mr. 

 Michelson himself confesses, it is impossible to study the effect 

 of the parallax due to the defect of coincidence of the plane of 

 the image with that of the lines of the micrometer ; in other 

 words, there is no defined focus. 



In all my experiments, therefore, it has been my aim to main- 

 tain the perfect accuracy of optical effects, such as had been 

 achieved by Foucault, believing that it is of greater advantage to 

 measure even the small deviations of a perfect image than the 

 exaggerated displacement of a blot of light. I have conse- 

 quently sought to amplify the deviations of Foucault without 

 increasing the distance to be traversed by the light, and without 

 having recourse to great velocities of rotation on the part of the 

 mirror. 



I call to mind, by the way, that Bessel noted, as a means of 

 increasing the deviation, the return of the deviated ray on the 

 rotating mirror. This method, which has never yet been 

 applied, might be utilised by means of a series of little plain 

 mirrors placed in couples on one >ide and the other of the 

 rotating mirror, in such a way as to transmit the pencil alterna- 

 tively on one and the other of the two parallel faces of the 

 rotating mirror. With each reflection the deviation increases 

 by a quantity equal to its primitive value. 1 But this process 

 would greatly complicate the measurement of the path traversed 

 by the light. The advantage contemplated by it may, beside , 

 be secured by a method much more elegant and simple. 



The apparatus I bring under the notice of the Academy con- 

 sists purely of two mirrors, one fixed, having a diameter of 

 0'20 m., the other movable, 005 m. in diameter, the two placed 

 at a distance of 5 m. from each other. Both are concave and 

 spherical, and have the same radius of curvature, 5 m. The 

 si an e 1 if light i a narrow aperture cut in the silver, in the centre 

 of the large mirror. The pencil emanating from it and entirely 

 covering the rotating mirror is reflected by the latter, and 

 returns to form on the surface of the fixed mirror a movable 

 image of the aperture and of the same size. In each of its posi- 

 tions this movable image becomes a source of light ; the rays 

 return to the movable mirror, which concentrates them anew 

 into a fixed image : this is the image of Foucault, which coin- 

 cides with the aperture when the rotation is very slow, which is 

 deviated in respect of the rotation when the latter is a little 

 rapid. Suppose the velocity of the rotating mirror to be such 

 that the lineal deviation is equal to the breadth itself of the 

 aperture, the image will then come to be formed on the fixed 

 mirror, rim to rim with the aperture itself. There it falls on 

 the reflecting surface of the silver, becomes then a source of 

 light exactly similar to the first, producing a second image 

 deviated by the same quantity. 1 he latter in its turn acts like 

 the fir t, in such a manner that, if one could look on the surface 

 of the fixed mirror, one would there be able to see, issuing from 

 the aperture itself, an indefinite series of identical images placed 

 rim to rim and indistinguishable from each other, except in 

 respect of their regularly increasing brightness. If the velocity 

 of rotation is increased, all these images will be found to separate 

 from each other and form on the fixed mirror a series of equal 

 luminous lines, separated by equal intervals from each other, and 



1 These plain mirrors, disposed in couples, might also be used to collect 

 and transmit in one constant direction the light scattered In all directions by 

 the rotating mirror. By this means the advantage would be obtained of 

 observing the doubled deviation of a much more brilliant image. 



