(uid Surface-Colour of NitrosO'dimethyl'aniline. 103 



'1 per cent, onlv, while a short distance further up the spec- 

 trum the coefficient is at least 80 per cent.* This indicates 

 that the refractive index has a very low value at the point of 

 mininiuni reflexion. The following table gives an idea of 

 the reflecting power of the nitroso in the various regions of the 

 spectrum as compared with the magnalium. The values are 

 of course only approximate, being deduced by picking out 

 two spectra (one from nitroso, the other from magnalium) 

 which showed the same intensity at the given wave-length, 

 and then comparing the times of exposure. The reflecting 

 power is expressed as a fractional part of the reflecting power 



of magnalium. 









X. 



Kefl. Power. 



X. 



Eefl. Power. 



217 



1/2 



380 



1/2 



240 



1/2 



399 



1 



290 



1/3 



467 



1 



330 



1/6 



508 



1/2 



343 



1/12 



535 



1/^ 



350 



1/40 



569 





When the incidence angle is increased and the reflected 

 light examined with a Nicol prism, a most beautiful surface- 

 colour appeared, ranging from a brilliant blue through violet 

 to a reddish purple. This is very easy to understand. The 

 violet light within the region occupied by the absorption- 

 band is metallically reflected at all angles of incidence, con- 

 sequently it is always present in excess in the reflected light, 

 and is never cut ofif by the nicol. As the angle of incidence 

 is increased, the polarizing angle for red light is first reached, 

 and the red end of the spectrum disappears, being cut off by 

 the nicol, the reflected light appearing blue. As the angle 

 increases more of the spectrum is removed by the nicol, and 

 the colour changes to deep violet, the dark band advancing 

 down the spectrum. Finally, by further increase the red 

 appears again in full intensity, being reflected unpolarized, 

 and mixing with the metallically reflected violet gives the 

 brilliant reddish purple. Very similar appearances can be 

 observed with selenium, though the colours are not as saturated 

 and not nearly so brilliant. 



Having roughed out, so to speak, the dispersion in the 

 ultra-violet by photographing the spectrum of the reflected 



* This vjilue is much too high. It was based on the supposition that 

 the magnalium reflected 80 per cent., which was subsequently found to 

 be incorrect, the metal containing* too little magnesium, and having a 

 rather low reflecting power. 



