132 ANNUAL OF SCIENTIFIC DISCOVERY. 



RESEARCHES ON THE INDICES OF REFRACTION. 



Jamin has undertaken to determine the refracting power of water when 

 compressed or when reduced to vapor. The experiments were executed by 

 means of the author's very beautiful apparatus for interferences, described in 

 the 42d volume of the Comptes Rendus. The water examined was enclosed 

 in two parallel tubes, one of which was open, while the other was subject to 

 variable pressure. At every change of pressure the fringes underwent a dis- 

 placement, which was measured, and from which the variations in the refract- 

 ing power of the liquid could be calculated. To avoid the error arising from 

 the increase in the length of the compressed column, the two tubes were 

 plunged into a trough full of water, so that the interfering rays traversed 

 the length of the tubes and the spaces separating their extremities from the 

 sides of the trough. If one of the tubes changes its length by a small quan- 

 tity, the external space diminishes by the same quantity, and thus the effect 

 of the dilatation is sensibly destroyed. The author finds that with this 

 apparatus one millimetre of pressure, more or less, produces an interval of 

 Y* - y- of a fringe, which is easily observed : for an entire atmosphere there 

 is a displacement of 28 fringes. The sensibility of the apparatus could be 

 still more increased by giving the tubes a greater length than that of one 

 metre which was employed. The author found that, in all his experiments, 

 the difference of path produced by pressure was sensibly proportional to the 

 pressure; so that if we calculate the compressibility of water from the opti- 

 cal experiments, we find the coefficient to be 0*0000500 for common distilled 

 water, and O'OOOOoll for water deprived of air. According to the direct 

 measures of Grassi, this coefficient is O'OOOOoOl. Jamin has also measured 

 with the same instrument the index of refraction for the vapor of water. 

 Two tubes Avere employed 4 metres in length: one of these was filled with 

 perfectly dry air; the other with air charged with a known proportion of the 

 vapor of water. The difference in the refractive powers could then be ob- 

 served by the change produced in the fringes. There was generally a differ- 

 ence of 8 fringes between dry and saturated air. More than fifty measure- 

 ments made under very different circumstances of temperature, pressure, 

 and hygrometric condition, agreed in assigning to the refractive power of 

 vapor at and 7HO mi " the value 0' 000-321. The author finds farther that 

 the diminution in the index of refraction of air by saturation with vapor 



would only affect the seventh decimal of the number 1 '000202 found 



for that index, and that, consequently, in astronomical refractions, it is useless 

 to trouble oneself about the vapor of Avater. Comptes Rendus, xlv. 892. 



NOTES ON THE SCINTILLATION OF STARS. 



The following is an abstract from a paper recently read before the Royal 

 Astronomical Society, England, by Prof. Dufour: 



Down to the year 18-32 no person, as far as I am aware, had undertaken 

 a serious of regular observations on the scintillation of the stars. Struck 

 with the difference Avhich the phenomenon presented from night to night, I 

 commenced in that year to observe it assiduously. At first it occurred to 

 me merely to make it a subject of meteorological inquiry; but I soon found 

 that the question was more complicated than I originally supposed it to be, 

 and that in any case, before entering upon its discussion, it would be ncces- 

 sarv to collect together a mass of observations extending over a considerable 



