﻿500 Prof. R. W. Wood on Fluorescence and 



is, it appeared to be tangled up in the sodium vapour, con- 

 densing with it in the cooler parts of the tube. All of this is, 

 however, irrelevant, for we are for the present merely engaged 

 with the study of a certain remarkable vibratory mechanism, 

 and for the present need not concern ourselves whether it is 

 a molecule, a cluster of molecules, or a compound molecule. 



We will begin by a description of the various spectra which 

 we shall study and compare in the present paper. The 

 spectrum region with which we are concerned lies between 

 wave-lengths 4600 and 5700, i. e. the region of the green- 

 blue channelled absorption spectrum. 



Hie Absorption Spectrum. 



Photographed with the twelve-foot concave grating, the 

 absorption spectrum is found to consist of a multitude of 

 fine lines, to the number of about 1500 in the region specified. 

 Its appearance has been found to be profoundly affected by 

 the presence of hydrogen or any inert gas. It is shown on 

 PI. XL (tig. 1, g), photographed in the second order with a 

 twelve-foot concave grating. In hydrogen at atmospheric 

 pressure its appearance is shown by spectra /and A, the fluted 

 appearance being very marked. In a high vacuum its 

 appearance is shown by spectrum Z>,the flutings having entirely 

 disappeared. The chief change appears to lie in the increased 

 absorbing power of certain lines or groups of lines. Spec- 

 trum d is taken under nearly the same conditions, the sodium 

 vapour being less dense, however. A careful study of the 

 effect of the inert gas has not yet been made, and it is 

 mentioned here only on account of its relation with the subject 

 of the paper. Comparatively few of the absorption-lines 

 have any relation with the fluorescent and magnetic rotation 

 spectra, the ones concerned, however, being those which are 

 affected by the presence of the inert gas. 



Moreover, both the fluorescence and magnetic rotation dis- 

 appear, i. e. cannot be excited, when the sodium vapour is 

 formed in an atmosphere of hydrogen or other inert gas. Of 

 this matter more will be said later on. 



The Magnetic Rotation Spectrum. 



It was found last year that a number of vapours, showing 

 fine and sharp absorption-lines, when placed in a powerful 

 magnetic field, rotate the plane of polarization for wave- 

 lengths agreeing with that of the absorption-lines, not all of 

 the absorption-lines showing this rotatory power, however. 



