Observations on various Liquids. 93 



and concave to each other. As the potential still rises, the 

 bands move symmetrically outwards from the axis, dividing 

 the flame into three large segments which are sensibly of equal 

 brightness. And when the electric action is at the strongest, 

 near spark-discharge through the liquid, the bands cross the 

 flame at points very little, if so much as, outside of the cylin- 

 der which envelops the two balls. The bands are still in- 

 tensely black and well-defined where they cross the flame; and 

 they are quite distinct in their whole course, as fine arches 

 resting on the two balls, and spanning the intermediate field. 

 When the machine is stopped, and the potential falls to zero 

 more or less slowly, the optical effect passes through the same 

 phases, but of course in reversed order. 



12. This fine experiment presents a case of what is known 

 in optics as the cross duplication of positive uniaxal plates, 

 the axis of the compensating slip being vertical, and that of 

 the liquid plate horizontal. The birefringent action of the 

 dielectric plate is not uniform either in time or space, but 

 increases in time at any given point of the field as the po- 

 tential rises, and diminishes at each instant regularly in space, 

 outwards from the axis of the electric field; and this con- 

 sideration affords a sufficient general explanation of the form 

 and phases of the phenomena. When the flame is divided 

 by the bands into three segments, the light of the middle 

 segment is restored by predominance of electric action in the 

 liquid plate, while the light of the extreme segments is still 

 restored by predominant action of the compensating plate of 

 glass. I think the following variation of the experiment 

 worth mentioning as an additional illustration of this view. 



The extinction-bands being formed as in (11), and kept 

 stationary in the outer parts of the electric field by constant 

 motion of the machine, I introduce the hand compensator, and 

 apply a horizontal compression beginning at zero. As the 

 compression rises gradually to a large intensity, the arched 

 bands move in gradually towards the axis of the field, until 

 they coincide in one band, which finally disappears. If the 

 compression be suddenly relieved at any point in this process 

 the arches come into view again at once in their old positions. 

 Like effects are obtained by a strong downward pull upon the 

 weight which is attached to the fixed compensator. 



13. These extinction-bands, obtained by electric action 

 against the tension of the fixed compensator, improve in all 

 respects with every increase of tension and corresponding in- 

 crease of potential. Against a tension of one pound or two 

 the bands are moved into the outer parts of the field by a 

 comparatively feeble electric action, and they are wide and 



