218 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
with pink and red streamers, and the long central tube with olive 
grey strata, apparently convex in both directions, but which the rotating 
mirror separated into two components, one convex in one direction, the other 
in the other. In the photograph only a continuous light is seen in the central 
tube, so that the detail, there shown, is from eye observation. 
Sensitiveness to External Influence. 
225. — In the preceding description we have alluded to the sensitiveness of tubes 
to the approach of the hand, or touch of the finger ; the tube shown in 
fig. 63, containing cyanogen, is an example of an extreme case : it gave 
tongue-shaped strata, and these entirely disappeared on the touch of a finger, 
all the tongues running into a continuous wave-discharge constricted near 
the finger. 
Eig. 63. 
Fig. 64. 
An endeavour was made on the suggestion of Professor Stokes to throw 
some light on the effect of external influence on the character of the strata, 
in the manner shown in fig. 64. 
