Intelligence and Miscellaneous Articles. 77 



value for the velocity of light so different from that which has 

 hitherto been accepted in science. 



It consists of a micrometric sight cut in a plate of silvered glass : 

 a rotating mirror supported on the axis of a small air turbine ; a 

 bellows with constant pressure; an achromatic objective; a series 

 of an odd number of concave spherical mirrors of silvered glass ; a 

 mirror inclined at partial reflexion ; a micrometric microscope ; and a 

 circular screen in the form of a toothed wheel set in motion by a 

 chronometric wheelwork. I shall first describe the apparatus at rest. 



A beam of solar light, reflected horizontally by a heliostat, falls 

 on the micrometric sight, which consists of a series of vertical lines at 

 a distance of -^ millim. from each other. This sight, which in the 

 experiment is the real standard of measure, has been divided with 

 great care by M. Froment. The rays which have traversed this 

 plane, impinge at a distance of a metre on a plane rotating mirror, 

 where they undergo a first reflexion, which sends them towards a 

 first concave mirror at a distance of 4 metres. Between these two 

 mirrors, and as near the plane mirror as possible, the object-glass is 

 placed, having on the one side the virtual image of the sight, and on 

 the other the concave mirror with its two conjugate foci. These 

 conditions being fulfilled, the beam of light, after having traversed 

 the object-glass, forms an image of the sight on the surface of this 

 first concave mirror. 



Hence the beam is reflected a second time in a direction so oblique 

 that it avoids the apparatus of the rotating mirror, the image of which 

 it forms in space at a certain distance. At the place at which this 

 image is produced a second concave mirror is placed, arranged so that 

 the pencil, once more reflected, passes near the first spherical mirror, 

 forming a second image of the sight ; this is taken up by a third con- 

 cave surface, and so on, until the last image of the sight is formed at 

 the surface of a last concave mirror of an odd number. I have been 

 able thus to employ as many as five mirrors, which develope a line 

 20 metres in length. The last of these mirrors is at a distance 

 of 4 metres from the last but one, which faces it, a distance equal 

 to its radius of curvature ; and it reflects the image on its own 

 path, a condition which is exactly fulfilled by superposing on the 

 surface of the opposite mirror the emergent image with the return 

 image ; that being effected, it is certain that the pencil reascends 

 the series, repasses by the plane mirror of the rotatory apparatus, and 

 that, finally, all the rays re-emerge by the sight, point by point, as 

 they entered. This fact of the return of the rays may be confirmed, 

 and an accessible image procured by turning, by partial reflexion, 

 on the surface of a mirror inclined at 45°, a portion of the pencil 

 which is examined with a feeble microscope. This latter, which in 

 all respects resembles the micrometric microscopes in use for astro- 

 nomical observation, forms with the sight and the inclined mirror a 

 very stable solid whole. 



In the apparatus thus described, the real image sent to the micro- 

 scope and formed by return rays partially reflected, occupies a 



