Electric Field on Emission Lines of Solids. 319 



were not long enough to bring out any other lines, though 

 according to Du Bois and Elias they should appear. 



At —180° sixteen other lines appear, on the side of longer 

 wave-lengths. The behaviour o£ the lines in a magnetic 

 field is very interesting. The very bright lines already 

 referred to split into unsy in metrical quadruplets. The centre 

 of the middle pair of lines in each group remains fixed, but 

 each group is unsymmetrical, as shown in fig. 4 of the plate. 

 The distance from the centre of the middle pair to the long 

 wave-length outer components is 0'25 A.U. more than the 

 distance to the short wave-length components. The total 

 separation of the outer components, with a field of 22,000 

 gauss, is 2'5 A.U. 



Most curious is the behaviour of the doublet 6988-5-6990*2. 

 As the field increases in strength this doublet widens slightly 

 and becomes fuzzy, then narrows down into a single line, 

 located at the centre of the initial doublet. (Fig. 4.) 



Of the other lines some appear to be uninfluenced by 

 the field and others disappear or become much blurred. 

 Photographs are reproduced on Plate IV. figs. 1 and 2. 

 Especially noticeable is the complete disappearance of the 

 lines between the main lines. Their behaviour in weak fields 

 indicates that their disappearance is due to broadening. 

 With a different orientation of the crystal the disappearance 

 does not take place in such weak fields, and the single line 

 which forms from the doublet is broader than in the case 

 of the other orientation. In all of these cases the light-rays 

 were parallel to the lines of force. There is, of course, need 

 of further study with different orientations. 



Resonance Radiation of the Ruby. 



Careful tests, using sunlight dispersed by a monochromatic 

 illuminator to excite with, showed that the strong red emission 

 lines are excited by light of their own wave-length. This, 

 or a similar effect in the case of a solid or liquid, has not 

 been before observed, so far as we are aware, and it is a 

 difficult thing to catch, since the fluorescence is much fainter 

 than when a broad spectral region is used to excite. How- 

 ever, there was no question that as the exciting light was 

 changed from the yellow through the red, the fluorescence 

 first gradually disappeared and then suddenly appeared 

 again as the wave-length of the line itself was passed. This 

 is indeed just what would be expected with a line showing- 

 selective absorption and therefore due to a normal mode 

 of vibration. 



Comparing the fluorite and ruby lines, then, we have one 



