466 Royal Institute of France. 
when the medium changes, all the other circumstances remaining 
the same, the absolute size of the deviations, and consequently 
the intervals of the fringes, vary also in proportion to the fits. 
The nature of the bodies which limit the medium do not change 
this law, whatever be the difference of their refrangent force. 
Stoppers of crown-glass opposed to each other form fringes in 
oil of turpentine, as metal stoppers would do if put in their place. 
Water at a heat of 37° of the centigrade thermometer forms 
fringes in water at 11°*, ; 
Sittings of Monday, March 11, 1816. 
In the name of a committee, M. Arago made a Report on 
the parallel mirrors which had been presented by Messrs. Richer. 
These glasses, which are of very delicate workmanship, are 
employed, as is well known, in the construction of reflecting in- 
struments, in that of tinned artificial horizons, which advan- 
tageously supply the place of the horizon of the sea in observa- 
tions which are made at land, and in the formation of the guards 
which serve to keep the liquids, by the help of which we also 
sometimes procure the reflected images of the stars free from 
those agitations which the least breath of wind makes them un- 
dergo. For a long period the English artists have been in the 
habit of supplying the French sextant-makers with parallel mir- 
rors. Messrs. Richer have succeeded in freeing us from this 
tribute. The glasses submitted to the examination of the Class 
were not less than four inches in diameter ; rarely have they ap- 
peared to occasion angular deviations of three seconds :—a si- 
Toilar glass, which a well known artist had recently purchased in 
London, under the same circumstances, afforded greater devia- 
tions. The plain mirrors of Messrs. Richer, like these which 
come out of the workshops of that excellent optician M. Re- 
bours, may therefore fairly be put on a level with all that has 
been done by foreigners of the same kind. 
M. Ivart gave a verbal account of several works upon agricul- 
ture, presented to the class by Sir John Sinclair. 
In the name of a committee, M. Giraud read a Report on a 
memoir of M. Dupin relative to the laying down of roads. 
* Messrs. Biot and Pouillet had undertalren these experiments at the 
end of the summer of 1815. On the 9th of October of the same year they 
announced to the Inssitnte that they had discovered laws, according to 
which the phenomenon of diffraction was found to have the most intin:ate 
connexion with that of the coloured rings, and might be deduced from 
them numerically. They had added that these laws indicated also the spe~ 
cies of modification extremely singular, according to which the light was 
diffracted. These indications are merely referred to the diflraction be- 
tween two stoppers, the only ones which the guthors had considered in 
their experiments, 
This 
