second. The number of revolutions of the wheel was determined acoustically in each experiment. The mirrors, at equal intervals on the periphery of the wheel, are inclined to the radius from R passing through the centre of each of them at an angle α = 29°. They are fixed solidly to R by screw movements capable of permitting a rigorous adjustment. The support for the bearings of the axle of R carries also fixed mirrors F, vertical like M, of which the number in the figure is three; but this number may, at will, be reduced, or increased up to nine. The position of the F's and M's is such that a parallel beam of light L, after a certain number of reflexions from the F's and M's (seven in the figure), may be received at L' when R has determinate angular positions. Naturally the intensity of L' is much weaker than that of L, and this enfeeblement is much more marked if R is in rotation, because in this case the light arrives at L' only during certain very short instants (ten times per revolution). I have observed in practice, however, that the four moving and three fixed reflexions of the figure allow of experimenting with light sufficiently intense at L' even if R is in motion: that is to say, that direct observation (without photography) suffices to establish the luminous phenomenon of which we have spoken above.
To study the value of λ the light L' was examined with the well-known interferometer of Michelson, shown diagrammatically in the figure. It is known that if the distances S1S3 and S2S3 are exactly equal fringes are observed with the telescope C even if the light is not monochromatic; these fringes then have the coloration of Newton's rings. As soon as a difference of path occurs (even if only of a few microns) observation with white light is no longer possible. Monochromatic light must then be used, and the order of the interference fringes increases with this difference. Their visibility is greater, the simpler the luminous vibrations. From the researches of Michelson[1] it is known that from this point of view the line that gives the greatest visibility of the fringes with the greatest difference in path is the green one of mercury (λ = 546μμ). In this case numberless circular fringes are visible even for a difference of path l = 2(S1S3 - S2S3) = 40 cm. I have therefore employed as source L a mercury arc in vacuo the light of which is conveniently filtered by solutions of chromate of potassium and chloride of nickel to absorb the violet and yellow rays;
- ↑ Travaux et Mémoires, Bur. Int. de poids et mésures, xi. p. 146 (1895).
