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378 On the refractive and dispersite Powers of certain Liquids: — 
braces a considerable number of bodies, we have felt the ne- 
eessity of extending our experiments still further, in order to en- | 
deavour to connect by some general law the various results which 
we obtained. 
The facts which we have presented appeared to us to be of 
such importance relative to the theory of light, that we have 
whought it would be useful to follow the consequences of it into 
the various phenomena which by their nature have a more or 
less direct union with that of refraction. 
The variously coloured rays of which white light is composed - 
are, as we know, unequally separated from each other by their re- 
refraction into bodies of a different nature, and in this consists the 
difference of dispersive power of bodies. What is most natural 
to take for the measurement of the dispersive power, is the dif- 
ference of the refracting powers relative to the extreme colours 
of the spectrum ; and in the Newtonian theory this difference — 
eught to be constant for one and the same body, as well as the — 
refracting power of the mean rays. ; 
Experience having shown that this last power diminished with — 
the density, it was easy to foresee that the dispersive power — 
would also diminish ; but it was important to examine if these _ 
variations followed the same law. In order to attain this object, 
it was necessary to determine the dispersive power of the liquids 
and vapours of which we had previously measured the refrac- 
ting power. The dispersive power of the liquids may be easily 
obtained, but it is not the same with that of the vapours. The 
refraction which they oceasion in a prism being very weak, the 
dispersion, which is only a very small part of this refraction; ag 
scarcely sensible. Thus, in spite of the importance of a similat 
determination either in the gases or in the vapours, experi- — 
mentalists seem to have given up the idea of deducing it from — 
ebservation. But as the object which we proposed to ourselves | 
required a direct measurement, we were constrained to attain — 
this object by the aid of a process of which we shall give a de- © 
tailed description. We shall find besides, from the results which _ 
we shall give, that the experiments made on one and the same _ 
yapour under different circumstances, agree well enough with | 
each other to entitle us to regard our determinations as ; closely d 
approaching the truth. 
On comparing the dispersive powers of the vapours thus mea-_ 
sured, with those of the liquid from which these vapours have pro- — 
ceeded, we were convinced that the dispersive power was effectu- _ 
ally diminished with the density ; but what observation has taught 
us, in a manner not less certain, is, that the dispersive power di- — 
minishes in a greater ratio than the refracting power; or in other 
words, by calling 7 the ratio of the sine of incidence to the 7 . 
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