﻿330 Prof. R. W. Wood on the Fl 



uorescence 



distance between the D lines of sodium was fully three 

 times their width. 



The magnetic-rotation spectrum which was discovered Inst 

 year, which manifests itself when polarized white light is 

 passed in succession through magnetized iodine vapour and 

 a Nicol prism, set originally for extinction, has been photo- 

 graphed with a concave grating of twelve feet radius, so that 

 a very perfect knowledge of its nature is now at our com- 

 mand. As is the case with the analogous magnetic-rotation 

 spectrum of sodium vapour, many of its bright lines coincide 

 in position with the dark lines of the absorption spectrum, 

 though the one is by no means the complement of the other. 



The remarkable emission of light which occurs when iodine 

 is raised to a temperature of 700° or over has been carefully 

 studied, and the condition found under which its spectrum is 

 very distinctly discontinuous. Its spectrum is remarkably 

 like the fluorescence spectrum, though the heads of the bands 

 do not appear to coincide, due possibly to a difference in the 

 densities in the two cases. 



The three types of spectra enumerated above will now be 

 considered in detail. 



The Fluorescence Spectmon. 



The fluorescence of iodine vapour manifests itself to the 

 best advantage when the vapour is formed in a moderately 

 high vacuum. A glass bulb from 5 to 10 cms. in diameter, 

 and containing a few flakes of iodine, is exhausted on a 

 mercurial pump and sealed. The fluorescence at once appears 

 when a beam of sun- or arc-light is focussed at the centre of 

 the bulb by means of a large condensing-lens, no heating of 

 the bulb being necessary. The cone of fluorescent light is 

 yellowish green and of considerable intensity, and can be 

 shown to a large audience in a darkened room. If the bulb 

 contains air at atmospheric pressure, there is absolutely no 

 trace whatever of fluorescence. Not until the pressure has 

 been reduced to about 15 cms. does the luminous cone appear, 

 and it is then so feeble that only close inspection reveals its 

 presence. The intensity gradually increases as the pressure 

 is further reduced, the most rapid change occurring between 

 1 cm. and the high vacuum. The presence of air also destroys 

 the magneto-rotatory power of the vapour, though to a less 

 degree at fairly low pressures ; in other words, the fluorescence 

 seems to be more sensitive than the magnetic rotation. I 

 was at first inclined to agree with Professor Larmor that the 

 effects might be due to collisions, the phenomena requiring 



