and the Dispersion of Opaque Bodies. 115 



tate, dissolved in 500 cubic centims. of water, form a liquid which 

 gives good results even when a great portion of the manganese 

 is separated by the electrolytic process. Concentrated solutions 

 are by no means applicable ; even the liquids which Nobili 

 (Pogg. Ann. vol. x.) and Bottger (vol. 1.) used for the preparation 

 of the coloured rings are too concentrated. A weakly-charged 

 element is used along with the decomposition-apparatus described 

 for hydrated peroxide of lead ; as the liquid is colourless, the 

 progress of the interference-bands during the operation can be 

 observed through the spectroscope. The strength of the cur- 

 rent is best directed so that the layer grows half a wave-length 

 within from fifteen to thirty minutes; if the growth goes on 

 too quickly, the thicker bright blue-black layer becomes brittle, 

 and with a change of temperature, especially by a wash of cold 

 water, easily cracks. 



To determine the specific gravity, layers of more than 100 

 wave-lengths in thickness and of the absolute weight of about 

 0*5 grm. were prepared; from two concordant experiments the 

 density at 13° C. was 



5=2-542. 



The body is not MnO 2 , but, like all bodies of this group, a 

 hydrate which does not lose its water under the air-pump. It 

 is transparent to green and blue rays in quite thin layers only, 

 of from 1 to 2 wave-lengths, so that minimum-bands can be ob- 

 served in E and F ; with greater thicknesses the same only appear 

 in the yellow and red. The minimum in F, however, is so broad 

 and faint that the wave-length cannot be determined from it by 

 the simple spectroscope without the application of photometrical 

 means. For the lines E, D, C, I have obtained according to the 

 foregoing methods the values 



n(E) = 1*944, 



n(D) = 1-862, 



»(C) = 1-801. 



III. General Conclusions. 



Besides the bodies already described, I also prepared a num- 

 ber of interference-layers in electrolytic and chemical ways, 

 which, like those described, are distinguished by an unusually 

 strong dispersion. The examination of these bodies has led to the 

 conclusion that all bodies of strong dispersion have optical pro- 

 perties in common which appear of interest for the theory of light. 



It is known from experience that dispersion and absorption 

 are related to one another ; and Cauchy, in his " Memoire sur la 

 dispersion de la Lumiere," has given an equation in which this 



12 



