1817.} Royal Institute of France. 155 
ROYAL INSTITUTE OF FRANCE. 
Account of the Labours of the Class of Mathematical and Physical 
Sciences of the Royal Institute of France during the Year 1815. 
Maruemaricat Part.—By M. le Chevalier Delambre, Perpetual 
Secretary. 
MEMOIRS APPROVED BY THE CLASS, 
ANALYSIS, 
(Continued from vol. viii. p. 462.) 
On the refractive and dispersive Powers of some Liquids, and the 
Vapours which they form. By MM. Arago and Petit. 
The theory of refraction is one of the most important parts of 
‘optics, and all the philosophers, who have explained or supported 
the different systems contrived to account for the phenomena, have 
particularly endeavoured to connect the law of refraction with the 
hypothesis which they admitted. 
Newton, in ascribing refraction to an attraction of bodies for 
light, has given so natural and clear an explanation of this pheno- 
menon, and of its Jaws, that it has been always considered as one 
of the principal arguments in favour of the system of emission. 
However, of all the general consequences deduced from this hypo~ 
thesis, the only one verified hitherto is the law of the constant ratio 
of the sines of incidence and refraction. But this law may be de- 
monstrated independently of attraction. Therefore, before adopting 
the hypothesis of Newton to the exclusion of all the others, it is 
necessary to examine how far the different conclusions resulting 
from it are confirmed by experiment. Such are the objects of the 
researches of which the memoir of MM. Arago and Petit gives the 
first part. 
The whole action of the body on light is measured by the incre- 
ment of the square of the velocity of the ray, and this increment 
may be denoted by the term refractive power. This quantity ought 
obviously to depend on the nature of the body. But in the same 
substance it ought to remain proportional to the density ; and 7 is 
natural to think that an attraction always acts proportionally to the 
mass, whatever be the function of the distance according io which it 
varies. On this supposition the refracting power, that is to say, the 
ratio of the refractive power to the density, ought to depend upon the 
chemical constitution of the body, and remain constant when the 
density alone changes. 
This consequence of the theory of refraction has never been 
verified, except in the gases. But their refractive power is very 
weak, the increase of velocity which they communicate to light is 
very small, and a very simple calculation will show that the ex- 
pression of the refractive power deduced from the Newtonian theory 
