and the Dispersion of Opaque Bodies. 109 



so forth'; and the differences m <2 —m l have for those cases the 

 values 1, 2, 3. With a little practice it is not difficult to deter- 

 mine by time the increase of the layer, and in this way to obtain 

 beforehand the moment in which the minima reappear in the 

 same place of the spectrum. 



If jo denotes the increase in weight of the layer determined by 

 the balance after a minimum has passed m times a given Fraun- 

 hofer's line, s the specific weight, o the surface of the layer, then 

 the refractive index of the substance for the line in question is 



m.s.o J a 



n ° — ^— » < 8 > 



when 4 denotes the wave-length of the light in air for that line. 



II. 



For the production of thin layers, with various substances several 

 methods often present themselves ; they cannot, however, always 

 be used for the production of good interference-layers. By 

 heating iron in air, for instance, very thin layers of protosesqui- 

 oxide of iron are obtained, a body which, prepared electrolytically, 

 gives distinct interference-spectra and beautiful colours. If it 

 be attempted, however, to strengthen by further heating the 

 thin layers which only show the first dull colours, we get 

 neither the beautiful colours of the second series, nor generally any 

 interference-bands in the spectroscope. The reason of this can 

 be easily shown : the layers obtained by heating are only pure 

 protosesquioxide on the surface ; within they get less oxygen ; 

 and from a body which from the outside to the inside is continu- 

 ally changing its refractive index the interference-phenomena 

 cannot be expected. 



The best interference-layers are obtained by suitable action of 

 chemical agents on thin metal layers, or by electrolysis. I have 

 prepared numerous interference-layers by both methods ; but at 

 present I confine myself to the latter, which is especially fitted 

 for oxygen compounds of the heavy metals. 



Nobili (Pogg. A nn. vol. x.) described a great number of liquids 

 the electrolysis of which gives coloured rings on plates of gold, 

 platinum, and other metals ; but they are of no value for the pro- 

 duction of interference-layers. It is easy to prove, for instance, 

 that the rings which he obtained on the positive electrodes of 

 silver, copper, zinc, and bismuth consisted of the oxygen com- 

 pounds of these metals, and were produced by oxidation of the 

 plates by means of the electrolyzed oxygen, but contained nothing 

 whatever of the substance of the electrolytes used. If Nobili 

 had used dilute sulphuric acid or an alkaline solution instead of 

 those different solutions, he would with a corresponding intensity 



