WOOD. 



NITROSO-DIMETHYL-ANILINR. 



55 



Nitrtfio. 



Figure A. 



ranging from 1.1 to about 1.5. I was subsequently able to use the 

 method of crossed prisms, by employing very acute prisms, kept warm 

 by means of an electrically heated platinum wire, which made long 

 exposures possible. 



I shall first, however, consider the polarization phenomena accompany- 

 ing the reflection of light from the surface of the fluid nitroso. The 

 substance was kept in a liquid 

 condition by means of a small 

 cell heated by a current of 

 steam. Figure A. 



On examining the light re- 

 flected at a fairly large angle 

 with a Nicol prism, it will be 

 found to vary from light blue to 

 deep violet and purple, as the 

 angle of incidence is increased, 

 the Nicol being held in such a 



position as to refuse transmission to the light polarized by reflection. If 

 a spectroscope is placed behind the Nicol, a dark band will be seen cross- 

 ing the spectrum, which shifts its position as the incidence angle varies. 

 The centre of this dark band is evidently the wave-length, for which the 

 angle of incidence happens to be the angle of maximum polarization, or 

 in other words the refractive index of the substance for this wave-length 

 is the tangent of the angle of incidence. In the case of glass and sub- 

 stances of low dispersion, the different colors are polarized at nearly the 

 same angle, i.e., very little color effect is observed when the reflected 

 lisrht is examined with the Nicol. In these cases the dark band to which 

 I have referred is so broad as to occupy practically the entire visible 

 spectrum. In the case of a substance with as high a dispersion as that of 

 the nitroso, the angle of maximum polarization is quite different for the 

 different colors, so that the dispersion may be determined by observing 

 the position of the centre of the dark band and the angle of incidence. 

 It was found that values agreeing very closely with those obtained with 

 the prisms could be obtained by this method in the yellow, green, and 

 greenish-blue portions of the spectrum. On attempting to drive the 

 band through the orange and into the red, it broadened so much that it 

 was quite impossible to locate its centre with any precision. This was 

 of course due to the fact that the dispersion in the red and orange is 

 not sufficient to make the method very accurate in this part of the 

 spectrum. The dispersion of selenium was also determined in this 



