78 DE. S. E. MILNEE ON THE NATUEE OF 



clearly separated, giving sharp-cut images, but the streamers corresponding to the 

 triplets still overlap and show a confused image with very little detail. Fig. 25 

 confirms the observations of SCHENCK,* who, also examining the magnesium spark 

 by a rotating mirror, observed that the streamers in it were associated almost entirely 

 with the spark line 4481, the arc lines giving only a diffuse luminescence which lasted 

 in the centre of the spark gap for some time after the actual discharge had ceased. 

 It would be incorrect, however, to infer that the arc lines take no part in the 

 production of streamers ; my results show that all the lines of a metal are exactly 

 equal in this respect, and that the clearness of the individual streamers is entirely a 

 question of the varying durations of the lines. By inserting enough inductance in 

 series with the spark, and thus separating the oscillations sufficiently on the screen, 

 the streamers corresponding to the arc lines can always be rendered clear. Thus in 

 ficr. 26, where the inductance in circuit has been increased, the streamers in the ultra- 

 violet triplet are distinctly shown. 



The difference which we have noted in the behaviour of the arc and the spark 

 lines of magnesium is characteristic of the whole of the metals examined. In every 

 photograph of the inductionless spark the arc lines last from four to six times as long 

 as the spai-k lines. With a little inductance inserted the spark lines always give rise 

 to sharp- and clear cut streamers, in sharp contrast to those of the arc lines, which 

 are diffuse and often overlap, producing a uniform luminosity which lasts some time 

 after the oscillations themselves have ceased. The streamers of the arc lines can, 

 however, always be rendered evident by inserting sufficient inductance in the circuit. 

 The number of oscillations registered on the plate is also generally a little greater for 

 the arc than for the spark lines, say about 14 oscillations as against 12. 



A detailed examination of the photographs showed that the spark lines themselves 

 are in many spectra divisible into two classes. We may take as an example fig. 14 

 (Plate 2) of the bismuth spark. In this figure the three diffuse drawn-out bauds 

 represent (counting from the top of the figure) the prominent arc lines 4722, 4122, 

 3 596, and the three sets of strongly-marked and clear-cut streamers lasting 

 throughout nearly the whole duration of the spark can be identified with the groups 

 of spark lines (5208, 5144, 5124), (4340, 4328, 4301, 4259), (3864, 3792, 3757). 

 In addition to these the lines 4560 (just below the highest arc band), 3695, and 3613 

 (at the bottom of the spark) can be seen. The first streamers of these lines show as 

 intense clearly-marked dots extending from the electrode only a short distance towards 

 the centre of the spark gap. The next streamers are much fainter, and after two or 

 three oscillations only no trace of the lines is visible. I have found lines of this class 

 only in the spectra of the metals aluminium, antimony, bismuth, lead, and tin. (It must 

 be remembered, however, that only the fairly strong lines of the metals can be observed 

 in the? photographs.) They are extremely sensitive to the influence of inductance in 

 the spark circuit, and disappear altogether from the spectrum when more than a very 



* Loc. at., p. 129. 



