C. Barns — Coronas with Mercury Light. 77 



eter of the inner edge of the first ring, are both observed, while 

 in parts VII and VIII data for s' and s'", the outer diameter 

 of the first ring, appear. 



In parts I and II (omitted) the goniometer was in front of 

 the fog chamber, and in series I a two-minute interval between 

 exhaustions was (exceptionally) introduced. The result is not 

 good ; for a sudden break of the curve appeared after the 

 seventh exhaustion, probably due to the time losses in the extra 

 minutes. The reason, however, is by no means obvious. In 

 series II, for one minute intervals there was no break and the 

 locus passing through the points for green discs (the others, not 

 marked g, are here and elsewhere to be disregarded) is per- 

 sistently straight throughout. The curves show 



For series I, ds / dz = 1 '00, and 

 For series II, ds / dz = '80, 



suggesting a time loss in the first case. 



Compared with the preceding table, the values of ds/dz 

 should be in the ratio of red and green minims, or 



ds r J dz : ds g /dz = *95 / -80 correspond to A r /\ = 63 -0 / 54'6 ; 



the last ratio, 1*15, is somewhat short of the former, 1*19. 



In series III and IV the pins of the goniometer are behind 

 the fog chamber, the eye being at the front wall. In series III 

 the relation, 



/ = 2-30 + 1-15 (19—2) 



is remarkably well sustained throughout ; and in series IY 



s' = 2-50 + 1-13 (18 — z) 



gives a good account of the green coronas, if the dull cases are 

 ignored. 



The most interesting results are given in parts Y and YI of 

 figure 1, and in the subsequent paragraphs the computations 

 have been fully carried out. In these cases the chords on a 

 radius of 30 cm of the edge of the green disc, s', and the inner 

 edge of the first green ring, s'\ were successively observed. 

 Figure 1 contains both pairs of curves and their linear character 

 is again astonishing. We may write 



Part V, s 7 =2-0-f 1-10 (19 — 2), Part VI, s'=2-0 + l*13 (18 — z), 



s"=3-4 + l-21 (19—2), s" = 3-3 + l-30 (18-3), 



s= -59 + 1 '06s' = — -56 + -96s", s= '50 + 1 -07s' — — -44 + -93s", 



where the minimum is located midway -between s' and s", both 

 of which are fairly sharp. 



From both series the mean value 



s = -55 + 1 -06 s'= — -50 + -93 s" 



