Discharge through a Coil of Variable Inductance . 155 



another instance of selective polarization mentioned in my 

 previous paper. The strongest metallic lines or gaseous lines 

 are not those which show the strongest reversal. For instance, 

 my previous photographs show the calcium line at approxi- 

 mately 4227 to be strongly reversed, while the stronger 3968, 

 3933 do not show a reversal except with much stronger and 

 longer continued discharges. 



A careful inspection of the negatives shows that the reversals 

 of the metallic lines occur when they fall on bright gaseous 

 lines or bands. In the same way a bright gaseous line falling 

 on a continuous spectrum can show a similar reversal. We 

 can express this in symbolic language as follows. Let A 

 represent the intensity of the line and B the amount of the 

 previous action of light on the photographic plate ; then the 

 reversal appears to be proportional to AB. 



It seems probable that there are similar reversed lines 

 running through the solar spectrum. I hope to detect them. 



This investigation shows that the presence of dark lines in 

 the spectra of stars dues not imply, necessarily, the presence 

 of reversing layers of a colder state of the gases ; for such 

 reversal may arise from photographic action on the plates 

 which are used. Moreover, a gas may show a continuous 

 spectrum to the eye, or even when photography is employed 

 with glass tubes and glass lenses, while with quartz tubes such 

 as I have employed a large region in the ultra-violet is shown 

 to be traversed by both dark and bright lines and bands. 



Jefferson Physical Laboratory, 



Harvard University, 



Cambridge, U.S. 



X. A Graphical Method of Determining the Nature of the 

 Oscillatory Discharge from a Condenser through a Coil of 

 Variable Inductance. By E. W. Marchant, D.Sc, late 

 Granville Scholar of the University of London, Lecturer in 

 Electrotechnics, .University College, Liverpool* . 



THE method which is described below is an extension of 

 the one originated by Dr. Sumpner (Phil. Mag. June 

 1887). 



In this paper he describes a graphical construction tor 

 determining the rate of increase of current in a coil having 

 an iron core. The problem is stated generally as the deter- 

 mination of the increase of current through a coil with a 



* Communicatee! by the Author. 



