Ixviii GENERAL SUMMARY OF SCIENTIFIC AND 



effect of the translatory motion of the earth on tire rgfrangi- 

 bility of light, in continuation of those made by Arago, and 

 with reference to Fresnel's theory. His apparatus was ar- 

 ranged underground, so as to be free from diurnal thermal 

 changes. The collimator was turned to the west, so that at 

 mid-day and at midnight the rays entering it would be mov- 

 ing, the one with the earth in direction, the other opposed to 

 it. A very numerous series of observations showed that the 

 change of deviation thus produced is entirely inappreciable, 

 and this with a perfection of methods which would detect a 

 twentieth part of that which Fresnel's formula supposes. 

 Indeed, in using mixed films, for example, Mascart shows 

 that the length of the apparent paths of the interfering rays 

 is not changed by this condition by one two-hundred-thou- 

 sandth part, that in observing Newton's rings it is not one 

 four-hundred-thousandth, and that in the fringes produced 

 by double refraction there is not produced by the movement 

 of the earth a change in the path of the two rays of one-mill- 

 ionth part. 



One of the most valuable optical discoveries of the year 

 is that made by Wolcott Gibbs, of a new physical constant, 

 which he calls the " interferential constant." It is well 

 known that when interference colors are viewed through a 

 prism a series of dark bands appears in the spectrum, known 

 as Talbot's bands. The number of these bands between any 

 two lines in the spectrum may be calculated when we know 

 the thickness of the plate producing the interference, the in- 

 dices of the given spectrum lines, and their wave-lengths. 

 If now the thickness of the plate be made unity, and the 

 formula thus modified be divided by the density of the sub- 

 stance composing the plate, an expression will be obtained 

 of a quantity called an " interferential constant." It repre- 

 sents the number of bands in the spectrum between two rays 

 whose indices are given, for a thickness of the plate equal to 

 a unit of density. This number is for each chemical sub- 

 stance a characteristic optical function, and independent of 

 the temperature. Its value will apparently be fully equal to 

 the other physical methods of analysis, such as density, boil- 

 ing point, specific volume, rotatory power, etc., while in some 

 examples given it finds important application in quantitative 

 analysis. Moreover, it appears that the interferential con- 



