392 On the Magnetic Action of Displacement-currents. [Feb. 28, 



It will be seen, then, that the continuity is practically perfect, both 

 as regards the intensities of the lines and the presence in each star of 

 the lines necessary for perfect continuity. 



VIII. Sequence of Spectra in Group III. 



The general sequence of spectra in passing from the earlier to the 

 later species of Group III is as follows, as far as the observations have 

 at present gone : 



(1.) The hydrogen lines are thin. D is thicker than b. The iron 

 fluting is faint. 499, E, 5327, 540, 568, and 579 are thin. 546 and 

 558 are fairly thick. 



(2.) The hydrogen lines arc thicker. E, D, and b are equally thick. 

 E, 5327, 540, 579, and 499 are much thicker, being nearly as strong 

 as E. The iron fluting has gone. 



(3.) The hydrogen lines are very much thicker than the other lines. 

 D and b are equally thick. E is nearly as strong as b, while the other 

 lines are fainter. 



(4.) The hydrogen lines are very broad, while all the remaining 

 lines are exceedingly faint. 



Subsequent work will no doubt enable us to further divide these 

 sub-groups into finer species. 



II. "On the Magnetic Action of Displacement- currents in a 

 Dielectric." By SlLVANUS P. THOMPSON, D.Sc., B.A. Com- 

 municated by Professor G. CAREY FOSTER, F.R.S. Received 

 February 19, 1889. 



(Abstract.) 



According to Maxwell's well-known views of electrostatic action, 

 the variations of electric displacement which occur during the charge 

 or discharge of a dielectric are to be regarded as equivalent to electric 

 currents. No direct experimental proof of this point has hitherto 

 been forthcoming. The author having calculated out on the assump- 

 tion of the equivalence between displacement-currents and conduction- 

 currents, what the effect would be of the charge or discharge of a 

 condenser upon a delicately astatised needle placed near the edge of 

 the condenser, concludes that the effects would be too delicate to be 

 measurable. He therefore resorted to a different method based upon 

 the principle that, if a closed curve be drawn around the flux of 

 electrostatic displacement, the line-integral of the magnetising force, 

 reckoned once round this closed curve, will at any instant be a 

 measure of the rate of change in the electric displacement through 

 the curve. Two forms of apparatus for realising this in an experi- 



