114 M. W. Wernicke on the Refractive Indices 



the passage of the interference-bands in the spectrum can be ob- 

 served and the current broken at the proper moment. The colours 

 form far more quickly than with suboxide of copper; the strength 

 of the current is so regulated that the layers increase half a wave- 

 length within from seven to ten minutes or a little longer. If 

 the increase takes place too quickly, the layers as they thicken 

 become brittle, and the colours are not perfectly pure. 



Whilst with suboxide of copper, for light having the refraction 

 of Fraunhofer's line F, the minimum-bands can be plainly ob- 

 served, yet with hydrated peroxide of lead this is no longer the 

 case for line E ; in the whole of the more refrangible portion of 

 the spectrum no trace of interference-bands is observable, whilst 

 in the yellow, and still more in the red part, they occur with 

 great sharpness. From this it must be inferred that the body, 

 even in layers of the thickness of one or a few wave-lengths, 

 is only transparent for yellow and red rays. 



To determine the refractive index for the line D, a thin plati- 

 num plate 13570 square millims. in surface was introduced into 

 the decomposition-apparatus, and after it had been tared the pas- 

 sage of a minimum-band was observed fifteen times through 

 that line. The increase in weight of this layer, fifteen half wave- 

 lengths thick, amounted to 0*166 milligrm.; therefore the re- 

 fractive index, according to formula (8), is 



15 x 13570 x 6-169 x 0-0005888 000 

 na ~ 2x0-1660 =3 ' 239 ' 



For the line C a similar inspection gave the value n(C) =2*010, 

 When the layer reaches a thickness of twenty-four half wave- 

 lengths (D), then from D downwards in the red part of the spec- 

 trum six minimum-bands appear, of which the last is almost coin- 

 cident with line B. Whilst a minimum travels from D to B, the 

 thickness of the layer increases about \ X (D) ; so that nineteen 

 wave-lengths X(B) = 23*5A(D). From this it follows that n(B) 

 = 1*802. The latter consideration may therefore serve as con- 

 trol for the direct determination by weighing. The numbers 



n (D) =2-229 



»(C) =2-010 



n(B) =1*802 

 show that the refraction by hydrated peroxide of lead is less, but 

 the dispersion somewhat greater than that by suboxide of copper. 



3. Hydrated Peroxide of Manganese. 



The interference-layers of this body are best obtained by elec- 

 trolysis of very dilute neutral solutions of salts of protoxide of 

 manganese ; 12 grms. chlorate of manganese and 8 grms. ace- 



