160 Prof. Pi. W. Wood. 



after traversing which the parallel rays passed lengthwise through the 

 dispersion tube. A second lens brought them to a focus on the slit of 

 a spectroscope, when the dispersion was to be studied by the method of 

 crossed prisms, or in the focus of an eye-piece when the anomalous 

 spectrum was to be viewed subjectively (fig. 2). 



The first experiments were made by the method of crossed prisms, 

 the spectrometer being furnished with a Rowland plane grating, which 

 showed the sodium lines widely separated. It was at once apparent 

 that far better results could be obtained with the dispersion tubes 

 than had ever been observed with prismatic flames. The curved 

 branches of the diffraction spectrum on each side of the D lines were 

 perfectly sharp and steady, and the dispersion could be traced to a 

 considerable distance up and down the spectrum. On the slit of the 

 spectrometer appeared, instead of the white image of the horizontal 

 slit, a most beautiful anomalous spectrum, of great brilliancy and 

 purity. The spectrometer was at once removed and an eye-piece put 

 in its place, when a most superb spectrum revealed itself. The general 

 appearance is shown in the coloured plate accompanying this paper, 

 though it is quite impossible to represent by means of pigments the 

 sparkling brilliancy of the colours. (See Plate 1.) 



Before discussing this spectrum in detail it will be better to take up 

 the results of the experiments made by the method of crossed prisms. 

 On first heating the tube the curvature of the spectrum between the 

 D lines as well as on each side is observed, the appearance being 

 identical with that figured by Becquerel, but in a few seconds the vapour 

 becomes so dense that total absorption of all the light between the lines 

 occurs. Julius expresses the opinion that this disappearance of the 

 light between the lines is only a result of the strong dispersion, that is, 

 it is not absorbed but turned off to one side so that it does not enter 

 the instrument. It appears to me, however, that this is not the case, 

 for I have observed the same effect under conditions where lateral 

 deflection seems quite out of the question. As I shall show later on 

 the breadth of the absorption band is sometimes twenty times the 

 width of the spectrum comprised within the D lines. 



The oppositely-curved branches adjacent to the region of absorption 

 grow out rapidly as the tube grows hotter, the ends finally passing 

 out of the field of the instrument. A beautiful fluted absorption 

 appears in the red and the greenish-blue, which finally blots out a 



