[ 375 J 
LXXVI. On the refractive and dispersive Powers of certain 
Liquids, and of the Vapours which they form. By Messrs. 
Araco and PETIT. 
Read before the Royal Institute of France \ith of December 
1815. 
Tus theory of refraction, when considered under the most 
general point of view, is one of the most important branches 
of optics, not only on acevunt of its numerous applications, 
but also from the consequences which may be deduced from it 
relative to the nature of light and to the true causes of its pro- 
perties. Thus the experimentalists who have developed or 
supported the various systems contrived for the explanation of 
the phznomena of optics, are always particularly anxious to - 
model the law of refraction according to the hypothesis which 
they adinit. 
Newton, in attributing refraction to an attraction of bodies for 
light, has given of this phenomenon, and of the law to which it 
is subjected, an explanation so clear and natural, that it has been 
always regarded as one of the principal arguments in favour of - 
the system of emission. Nevertheless, if we remark that of all 
the general consequences deduced from the hypothesis of New= 
ton, the only one which to this day has been verified is reduced 
to the law of the constant ratio of the sine of incidence and of 
refraction ; if we observe besides that this law may be demon- 
strated without its being necessary to have recourse to the idea of 
an attraction, we shall easily perceive that before determining 
to adopt the hypothesis of Newton to the exclusion of all others, 
it is indispensable to examine to what extent the various con- 
elasions which flow from it are confirmed by experience :—such 
is the object of our present researches, In order to know their 
object correctly, it will be necessary to retrace in few words 
the principal points of the theory of refraction, such as Newton 
has inferred from the supposition of an attraction exercised by 
bodies upon light. 
On this hypothesis, we may easily conceive that when the 
molecules of which a ray is composed approach the refracting 
body, the attraction which it exercises on them changes both 
their velocity and the direction of their motion, and that this mo- 
tion becomes once more uniform and rectilinear, when the mo- 
lecules have penetrated into the body to the depth where the at- 
traction ceases to be sensible. The principle of active forces: 
applicable in this case, proves that the velocity which the light 
has acquired by the effect of refraction, is independent of 
the initial direction of the ray, and that the ratio of this ve- 
Aa4 lovity 
