448 Astronomical and Nautical Collections. 



it is very easy to observe the difference of one fifth of a 

 fringe, which corresponds to a retardation of one fifth of an 

 undulation in one of the pencils, that is, the ten millionth 

 part of the whole length of 42 inches ; we might therefore, 

 by introducing any gas or vapour into a tube of this length, 

 terminated by two plane glasses, estimate very accurately the 

 variation of its refractive power. 



I take the length of an undulation of the yellow rays, 

 which are the most brilliant of the spectrum, and of which 

 the dark and light stripes consequently coincide with the 

 darkest and brightest stripes of the fringes produced by 

 white light, which is commonly employed in these experi- 

 ments, both because of its greater brightness, and because of 

 the more marked character which it gives to the central stripe, 

 so as to prevent any other from being mistaken for it. 

 . It was an apparatus of this kind that Mr. Arago and 

 myself employed for measuring the difference of the refrac- 

 tive powers of dry air, and of air saturated with moisture at 

 80° F., which is so small, that it would escape every other 

 method of observation, because the greater refractive power 

 of aqueous vapour is almost exactly compensated by the less 

 specific gravity of moist air. But, in the generality of cases, 

 the slightest mixture of one vapour or gas with another 

 produces a considerable displacement in the fringes : and if 

 we had a series of experiments of this kind, made with care, 

 the apparatus might become a valuable instrument of chemi- 

 cal analysis. 



[To be continued.] 



iii. Remarks on the Action of ConvvscvLAuFoncES. In 

 a Letter to Mr. Poisson. 



My dear Sir, 



I AM very glad to see that you have been applying your 

 analytical powers to the investigation of the acustical effects 

 of corpuscular forces, and that, among many more refined de- 

 terminations, you have confirmed several of the results relat- 

 ing to sounding bodies, which were published twenty years 

 ago in my Lectures on Natural Philosophy : though they 

 were generally such as might have been derived from the cal- 

 culations of Bernoulli and Euler ; which I attempted in some 



