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 XLVIII. Intelligence and Miscellaneous Articles, 



ON THE ANOMALOUS DISPERSION EXHIBITED BY CERTAIN 

 SUBSTANCES. BY M. J. L. SORET. 



MCHRISTIASEN and M.Kundt have recently published some 

 • very remarkable experiments upon the anomalous dispersion 

 that certain substances, such as the aniline colours, permanganate of 

 potash, and others, exhibit when examined in concentrated solutions. 

 A prism formed of these fluids between two plates of glass shows a 

 spectrum wherein the sequence of the colours differs from that of 

 ordinary substances, blue and violet being less refracted than red. 

 When performed in this manner the experiment presents a certain 

 difficulty, inasmuch as the fluids are of a very deep colour, and the 

 light can therefore pass through but a very limited thickness thereof, 

 so that the pencil can only be allowed to traverse the fluids quite 

 close to the edge of the prism. 



The importance of the phenomenon in question will not have 

 escaped the notice of any physicist ; and I therefore conceive it may 

 prove of some interest to specify a consequence thereof that may be 

 rendered evident by an experiment which can be repeated without 

 difliculty. It consists in the inversion of the spectrum. This is 

 efl^ected by putting the solution under examination into a hollow 

 prism of about 30°, and placing this prism in a cell with sides of 

 parallel glass, which is tilled with the fluid used for forming the 

 solution of the substance with anomalous dispersion experimented 

 upon. One can then observe the inverted spectrum in a less con- 

 centrated, and consequently more transparent solution, than when 

 the prism remains in air. I will cite a few instances. 



Fuchsine, or Magenta. — Take a spectroscope, and having removed 

 the ordinary prism, replace it by a hollow prism filled with a con- 

 centrated solution of fuchsine. If one employs an intense pencil 

 of light falling very near the edge of the prism, one succeeds in 

 getting a sight of the inverted spectrum without having recourse to 

 the cell spoken of above — that is to say, when the prism is left in air. 

 With a solution of fuchsine that is only moderately concentrated 

 the spectrum is normal, or, in other words, the red is less refracted 

 than the violet. With a solution of intermediate strength, the spec- 

 trum becomes reduced almost to a single bright red-coloured band ; 

 in this case the inverted dispersion due to the fuchsine is almost 

 exactly balanced by the opposite dispersion brought about by the 

 alcohol employed as the solvent; we have deflection without 

 dispersion. 



If we now remove the prism filled with this latter solution into 

 the cell containing alcohol, the general deflection of the rays is nearly 

 entirely annulled, while the anomalous dispersion of the fuchsine 

 remains ; the deflection of the red exceeds that of the violet. It is 

 no longer required to employ so intense a light, or to let it fall close 

 to the edge of the prism. 



On measuring the angle of deflection for the solution in question, 

 I found, when the prism was in air, that it amounted, for the red 

 band, to about 11° 30', but when put into alcohol there was scai'cely 



