396 Intelligence and Miscellaneous Articles* 



any deflection of the violet, while that of the red was 15', and that 

 of the orange-yellow 23', With a far less concentrated solution of 

 fuchsine the deflection of the violet was also nearly imperceptible, 

 while that of the red was 6' and of the orange-yellow 16'. 



Hence, for the violet rays, solutions of fuchsine have almost the 

 same refractive index as alcohol, but for the red rays a higher one. 



Aniline violet. — With an aqueous solution of this substance, and 

 with the prism situated in air, I obtained a normal spectrum, wherein 

 all the colours were visible ; the deflection of the red was 10° 24', and 

 that of the violet 10° 43'. On placing the prism in the cell filled 

 with water, the spectrum became reduced to two bands (one blue, and 

 one of a carmine red), which overlapped when the ?lit of the spec- 

 troscope was not extremely narrow. When sunlight was made use 

 of, there was, in addition, a trace of green towards the end of the 

 spectrum at the side of the blue band. The deflection of the blue 

 amounted to 1', that of the red to 4'. 



Permanganate of Potash. — When filled with a solution of per- 

 manganate of potash, the prism, in air, gave a normal spectrum, 

 wherein the deflection of the red was 10° 33', and that of the violet 

 10° 53'. When placed in water, the prism gave, for violet, a deflec- 

 tion of 6', for red of 9', and for yellow of 12'. 



From these numbers we perceive how the addition of substances 

 having anomalous dispersion lessens the dispersive power of the 

 solvent without materially affecting its mean refractive index. If 

 one goes on increasing the strength of the solution, the dispersive 

 power becomes at first zero, and then negative. The experiment 

 with the prism surrounded with the solvent is of higher interest with 

 the substances last named than in the case of fuchsine, inasmuch as 

 aniline violet and permanganate of potash must be used in extremely 

 concentrated solutions if they are to exhibit an inverted spectrum with 

 the prism in air. In this case the observation is far more difficult to 

 make than with fuchsine. 



The papers of Messrs. Christiasen and Kundt, referred to above, 

 are to be found in Poggendorflfs Annalen, vol. cxliii. p. 250, vol. cxlii. 

 p. 163, and vol. cxliii. pp. 149 & 259. Vol. cxliv. p. 128, vol. cxlv. 

 pp. 67 & 164, vol. cxlvi. p. 154 contain further papers on the same 

 subject. — Poggendorflfs Annalen, vol. cxliii. p. 325. 



ON THE MEASUREMENT OF THE INTENSITY OF CURRENTS BY 

 MEANS OP THE ELECTROMETER. BY M. E. BRANLY. 



When a wire is traversed by a voltaic current, its diflPerent sec- 

 tions present dififerent charges of static electricity. A and B being 

 two points in the conductor, a the electric potential in A, b the po- 

 tential in B, the difference a — b is proportional to the intensity of 

 the current. This is expressed by Ohm's formula 



r 



C is a constant ; r represents the resistance of the part AB. If the 

 wire is cylindric and homogeneous, the electrical density at each 



