PHYSICS. 601 



and the fringes are observed, with a telescope. The results obtained 

 between 24 and 36 atmospheres are now given. For the number of the 

 fringes which would pass under the reticule for a variation of pressure 

 of 1 millimeter in a tube 1 meter long, the author obtained 0.550, 0.510, 

 and 0.555 in three experiments, the number calculated on adopting the 

 index for air at 22° the value 0.000271, being 556. (G. B., March, 1883, 

 XCVT, 699; PHI. Mag., April, 1883, Y, XV, 299.) 



Sarazin has determined with great care the ordinary and extraordinary 

 indices of Iceland spar for the leading Fraunhofer lines as well as for 

 twenty-six of the leading lines of cadmium. He used two prisms, one 

 by Hofmann, the other by Laurent. (J. Phys., August, 1883, II, ii, 369.) 



Loewy has given a detailed description of the novel telescopic mount- 

 ing devised by him for the new equatorial of the Paris Observatory, and 

 constructed by Eichens and Gautier. (J. Phys., August, 1883, II, n, 

 349.) 



Thollon has constructed a modified form of collimating telescope for 

 use with his liquid prisms, which must be kept in a horizontal position. 

 If atotal reflection prism be placed behind the slit of a collimator, so that 

 its hypotenuse is at the same time parallel to the axis of the telescope and 

 to the slit, an image of the slit will be formed inverted, as in Zollner's 

 reversion prism. If the slit be turned through any angle, and the prism 

 through half this angle, an image of the slit will be given which will 

 coincide in direction with the first image. This device works well in 

 practice. (G. R., March, 1883, xovi, 642.) 



Tait has discussed that state of the atmosphere which produces the 

 forms of mirage observed by Vince and by Scoresby. From theoretical 

 considerations he concludes that the conditions requisite for the pro- 

 duction of Vince's phenomenon, at least in the way conjectured by him, 

 are a stratum in which the refractive index diminishes upward to a 

 nearly stationary state, and below it a stratum, in which the upward 

 diminution is either less or vanishes all together. The former condition 

 secures the upper erect image, the latter the inverted image and the 

 lower direct image. In proof of the correctness of this theory, the au- 

 thor constructed a tank with parallel glass ends, about 4 feet long, and 

 filled it one-half with a weak brine carefully filtered. Pure water was 

 then cautiously introduced above it till the tank was nearly full. After 

 a few hours the whole had settled down into a state of slow and steady 

 diffusion, and Yince's phenomenon was beautifully shown. (Nature, 

 May, 1883, xxvin, 84.) 



3. Dispersion and Color. 



Zenger has produced a direct vision prism of great dispersive power 

 by joining to a dispersion parallelopiped a prism of light crown. He 

 claims that by this combination he gets a dispersion of 150° between 

 the A and the H lines. Since ordinary direct vision prisms give a sep- 

 aration of these lines of only 20°, this result is remarkable and ex- 



