324 



Royal Society : — 



to me, some rough drawings of which I lay herewith before the 

 Royal Society. As the phenomena are only momentary, I cannot 

 answer for the final accuracy of the drawings, nor have I been able 

 to represent the softness of the gradations of shade. 



2. In the drawings, the red end of the spectrum is to the left ; 

 the D line common to them all is the image of a slit about half an 

 inch long, on which slit the light falls from an electric lamp, through 

 the tube and chamber in which the vapours are produced. The 

 lower part of the drawings would generally represent, therefore, 

 the spectrum of the less dense vapours were the vapours at rest. 



3. One of the phenomena referred to consists of what may be 

 described as a unilateral widening of the line D : the side absorp- 

 tion, however, is much less dense than that of the line ; it is bounded 

 by D on one side and by a curved line on the other. Figs. 1, 2, 

 and 3 will give an idea of this appearance in three stages as it is 

 frequently actually seen, i. e. as the absorption travels up or down 

 the line it widens as shown. 



Fig.l. 





Kg. 2 



Fig. 3. 



4. Figs. 4 and 5 give two variations sometimes observed — fig. 4 

 showing the darkening in the absorption and an increased steepness 

 in the curve ; fig. 5 the simultaneous existence of apparently dif- 

 ferent absorptions, all bounded by D on one side, but by different 

 curves on the other, and being of different intensities. 



