5 73 Roy^l Society. 



tJaut»iriJ 2<u4'^"f £'i iB'Jiiisv 3ih nwH i9vi E and K,,, ,^ ^^^ j^jy- 



^ C and II, G and M ^^,niem9? 



D and I, . . i ,. j. i. .<i,,...iv/ 



shows that tlie kngths of undulation for the saijie colour in the air 

 and in the glass are in the ratio l:')5 to 1, which is exactly the index 

 of retraction of this glass. He further remarks, that, as the same 

 period of undulation belongs to the same colour, the velocities of 

 propagation iri air and in tlie glass must be in the ratio of the mean 

 value of the distances of the lines in the first seven systems, A, B, C, 

 D, E, F, G, to the mean value of the distances of the lines in the 

 last seven systems, F, G, H, I, K, L, M, or as 1-53 to 1 ; and that 

 both results agree perfectly with the deductions from the wave- 

 theory of light. In conclusion, the author points out the extraordi- 

 nary degree of accui-acy required in drawing these Hues. It is stated 

 that if inequalities amounting only to '000002 line occur in the sy- 

 stems F, G, H, I, K, L, M, stripes of another colour will appear in 

 them ; and if the distance of the Itijes in M be diminished by that 

 quantity, all colour disappears. ; : 



"Note relating to M. Fouoault's new luephanical proof of the 

 Rotation of the P^arth*." By C. Wheatstcne, F.RjS., Corresponding 

 Member of tlie Academies of Science of Paris, Brussels, Turin, 

 Dublin, &c. 



The experiment made by M. Foucault is said, both in the direction 

 and magnitude of the motion of tiie plane of oscillation of the pen- 

 dulum, fully to coiitirm the indications of theory. The difficulty, 

 however, of the mathematical investigation of the subject, and the 

 delicacy of the experiment, liable as it is to so many extraneous 

 causes "of error, have induced many persons to doubt either the 

 reality of the plia'nomenon or the satisfactoriness of the explanation. 

 Another experimental proof, there lore, not depending on the rotation 

 of the earth, that tiie plane of oscillation of a vibrating line remaiiw 

 at rest with relation to the vertical component of the real axis of 

 rotation, may not be unacceptable. With this view I have deVised 

 the api)aratus I am about to describe. ■ ■ , '- 



A scmicircnhir arch from one to two feet radius is fixed vertically 

 on a horizontal wheel, and may thus be moved with any degree of 

 rapidity from any one azimuth to another. A rider slides along the 

 inner edge of the arch, which is graduated, and may be fixed at any 

 degree marked thereon. A spiral spring wire, by means of which a 

 slow vibration is obtained witii comparatively a short length, is at- 

 tached at one end to a pin fixed in the axis of the semicircle, so that 

 the point of attachment may be in the axis of rotation, and at the 

 other end it is fixed to a similar pin in a pjirallel position fixed to 

 the rider. Tiie vertical semicircle is not placed in a diameter of the 

 horizontal wheel, but parallel to it, at such distance as not to inter- 



* This pajyer \vas rend to the Society on the 2:2u(l of May ; we give it 

 immediate insertion, deferring absti-aets of j)receding papers, in order to 

 render the notices on M. Foucault's interesting experiment contained in 

 our |)resent Number tlic more complete. — Eds. 



