M. Mascart on the Achromatism of Interferences, 519 



Isle of Wight. 

 Barton Clay. Headon Hill Sand. 

 Wanting. 

 Lower Headon. 

 Middle and Upper Headon. 



Osborne. 



Bembridge Limestone. 



Bembridge Oyster-bed. 

 f Bembridge and 

 1 Hamstead Marls. 



Hamstead Corbula-beds. 



It will be seen that the number of alternations of strata is 

 about the same in the synchronous formations in the Paris 

 and Hampshire basins. This shows that this alternation of 

 strata was due to a general cause, and that this cause is the 

 precession of the equinoxes seems highly probable. 



As, moreover, the curve of the eccentricity of the earth's 

 orbit appears at the same time to be a curve of the variations 

 of the sea-level, we may also conclude with probability that 

 for one reason or another the sea rose and fell with the 

 eccentricity. 



Paris. 



Gres de Beauchamp, &c. 



Calcaire de St. Ouen. 



Sables de Monceaux. 



Marne a Pholadomya, Grypse 



No. 3, Marne a Lucina. 

 Grypse No. 2-1. Marne bleue. 

 Marue blanche. 

 Marne verte. 



Calcaire de Brie. 



Marne et Mollasse marine. 



LVIII. On the Achromatism of Interferences. 

 By M. Mascart*. 



1. TN a communication to the Academie des Sciences, M. 



J- Comuf investigated the phenomenon of the displace- 

 ment of interference-fringes in white light by the interposition 

 in the path of one of the rays of a refracting plate, and showed 

 that the new position in which the central fringe appears de- 

 pends not only on the optical retardation, but also on the dis- 

 persion of the interposed medium. He applied the name 

 " achromatic fringe '" to that corresponding to the same phase 

 for the most important colours of the spectrum. 



Prof. Stokes f had already pointed out this property, and 

 had proved by experiment that observing the fringes through 

 a prism of small angle produces an apparent displacement of 

 the fringe from symmetry. 



It should also be added that, since the first application 

 of the method of fringe-displacement to the measurement of 

 indices of refraction, Fresnel § recognized that it was in- 



* Translated from the ComjJtes Rendus, March 25, 1889, p. 591. 

 t Comj>te8 Rendiis, xciii. p. 809 (1881). 

 X Brit. Assoc. Rep. pt. 2, p. '2Q (1850). 

 § (Euvres de Fresnel, ii. p. 268. 



