MORSE, — SPECTRA OF WEHNELT. 543 



It seems probable that the environment of the point does pass through 

 a very great range of temperature with each interruption of the current. 

 There seems to be no reason for believing that the same is not true of 

 the spark in air and of the arc. In the spark in air the discontinuity 

 of succeeding oscillations is quite complete, and the necessity for an ex- 

 planation of the absence of lines ascribed to lower temperatures seems 

 as evident as in the case at hand. It seems probable that all three of 

 these spectra, spark, arc, and Wehnelt, are composites, though this is 

 not so evident in the usual methods of spectrum production. 



In the Wehnelt, reversals were not observed in any case. This is 

 remarkable since Hale,* and Hale and Kent,t and Lockyer, t and others 

 who have examined the spectra of the arc under liquids have shown that 

 a very strong tendency toward reversal may exist under these circum- 

 stances. The mechanism of spectrum production in the two cases must 

 be very different. 



Lockyer gives as the result of his investigations and as the basis of 

 many deductions four distinct stages of temperature corresponding to 

 the spectra of metals as they are produced in the laboratory. They are : 

 " 1. The flame spectrum, consisting of a few lines and flutings only, 

 including several well-marked lines, some of them arranged in triplets. 

 " 2. The arc spectrum, consisting of many lines. 



"3. The spark spectrum, differing from the arc spectrum in the en- 

 hancement of some of the short lines and the reduced relative brightness 

 of others. 



"4. A spectrum consisting of a relatively very small number of lines 

 which are enhanced in the spark. This latter spectrum is produced at 

 a temperature which is that of the very centre of the spark." 



Lockyer has also made frequent use in his writings of a diagram show- 

 ing a series of furnaces of increasing temperature, spectra being used to 

 designate the temperature in each. The flame is taken as the lowest, 

 the arc next, and then the spark, followed in application to astrophysical 

 comparisons by the spectrum of a typical cooler star, then hotter ones, 

 and so on. We have in the Wehnelt interrupter a " furnace " in which 

 under constant experimental conditions a banded (compound) spectrum 

 like that found for aluminium, and a pure line spectrum like that found 

 for platinum may be produced. The amount of current passing in the 

 two cases is the same, the other factors of the experiment (composition 



* Astropbys. J., 15, 190 (1902). 

 t Ibid., 17, 155 (1903). 

 t Ibid., 15, 190 (1902). 



