340 Intelligence and Miscellaneous Articles. 



of the two mirrors or the two halves of the prism. Fresnel (Mcmoires 

 sur la (I iff radian, vol. i. p. 355 et passim) assumes that, in the central 

 part of the fringes, the diffraction resulting from this limitation of 

 the waves plays but an absolutely negligible part. On that hypo- 

 thesis the fringes should all have the same breadth, proportional to 

 the distance from the focal plane to the luminous images, the minima 

 of intensity should all be nil, and all the maxima equal, in homoge- 

 neous light, and in white light the central fringe should be of ab- 

 solute whiteness. 



According to M. Weber *, attentive observation shows that these 

 consequences are not exactly verified : the relative breadths of the 

 lines vary with the distance of the sources from the focal plane in 

 which they are observed ; the intensities of the maxima and minima 

 differ very much ; in white light the central line is nearly always 

 coloured. M. Weber arrives at the explanation of all these appear- 

 ances by taking into account the fact that the waves which produce 

 the phenomenon are not indefinite. By calculations necessarily 

 long, and into the details of which we of course cannot enter, he 

 first reduces the problem to the determination of Eresnel integrals, 

 which he afterwards expresses by means of a Bessel function and 

 another, analogous, definite integral. He thus obtains a compara- 

 tively simple expression for the intensity of the light in any point 

 of the interference-field. — Bibliotheque Universelle, Archives cles 

 Sciences physiques et naturelles, September 15, 1879, tome ii. 

 pp. 360, 361. 



AN ABSORPTION HYGROMETER. BY A. VAN HASSELT. 



The air whose moisture is to be ascertained is introduced into a 

 flask which is furnished with a thrice-perforated stopper. Through 

 one of the perforations a glass tube is passed which reaches to the 

 bottom, through the second a thermometer, and through the third 

 a glass tube communicating, by means of an indian-rubber tube, 

 with a manometer having oil for its liquid. Both the glass tubes 

 can be closed by glass cocks. The indian-rubber tube is itself sur- 

 rounded with a second one, filled with oil in order to prevent any 

 diffusion of the aqueous vapour — a circumstance which Edelmann 

 (Wied. Ann. v. p. 455) has not taken into consideration. It might 

 probably be advantageous to substitute glass for the indian rubber. 

 I'irst, a thin-walled glass sphere containing anhydrous phosphoric 

 acid is put into the flask, and is broken by shaking the latter. 

 From the variations of pressure in the manometer the amount of 

 aqueous vapour is determined. In order that the previous volume 

 of the air may be restored, the manometer consists of two glass 

 tubes connected by one of indian rubber. The results were per- 

 fectly satisfactory. — Beibldtter zu den Annalen der Physik und 

 Chemie, 1879, No. 9, p. 697. 



* Vierteljahrsschrift der Ziiricher naturforschenden Gesellschaft, 1879. 



