and Surface- Col our of ^itrosO'dimethyi-amUne, 97 



In the case of the so-called transparent substances, the 

 absorption-band lies so far down in the ultra-violet that the 

 steepness o£ the dispersion-curve in the visible spectrum is 

 not comparable with that of the substances which are said to 

 exhibit anomalous dispersion. If. however, we push the 

 curve down to the absorption-band, employing some photo- 

 graphic method, we find that it may be even steeper than the 

 curve for cyanine in the red and orange. 



I have found that nitroso-dimethyl-aniline is of peculiar 

 interest, in that it fills in the gap existing between the aniline 

 dyes and ordinary transparent substances. It has a band of 

 metallic absorption in the violet, and is at the same time 

 fairly transparent to the red, yellow, and green. The sub- 

 stance melts at 85^ C, and can be formed into prisms between 

 small strips of thin plate-glass. The strips should be about 

 2 cms. long, and are best fastened too-ether with one of the 

 small clamps used with rubber tubing. It is best to melt 

 the material on the end of one of the strips, the other being- 

 warmed over the same flame, and then clamp the two together 

 with a piece of a match between the other ends, to give the 

 required prismatic form. A caudle-flame viewed through 

 the prism is spread out into a most remarkable spectrum 

 fifteen or twenty times as long as one given by a glass prism 

 of the same angle. It is instructive to have a prism of the 

 same angle made of Canada balsam or some such substance 

 pressed out between two similar glass strips. 



In addition to its remarkable dispersion, the nitroso, as I 

 shall call it for short, exhibits a most beautiful violet surface- 

 colour, which can best be exhibited by employing a small 

 cell heated by steam such as I shall describe later on. 



The substance possesses iu addition several other interesting- 

 features. It has, for example, in addition to its sharp and 

 narrow baud of metallic absorption in the violet, a weaker 

 pair of bands near the end of the ultra-violet which flatten 

 out the dispersion-curve, but do not bend it into oppositely 

 directed branches as the stronger band does. Moreover, 

 the substance can be vaporized without decomposition^ which 

 makes it possible to compare its optical properties in the three 

 states — solid, liquid, and gaseous. 



I shall take up in order the dispersion in the visible spec- 

 trum, the ultra-violet dispersion, the reflecting power in 

 difPerent parts of the spectrum, the angles of maximum pola- 

 rization, and the changes in the position of the absorption- 

 band which accompany a change of state. The various 

 results will finally be discussed in their bearing on the electro- 

 magnetic theory of dispersion and absorption. 



Phil. Mag. S. 6. Vol. 6. No. 31. July 1903. H 



