1894.] Fundamental Question in Electro- Optics. 



Final Experiments with CS 3 . The first internal conductor con- 

 nected permanently with prime conductor without Leyden jar, the 

 liquid quite clean, and the conditions of electric work perfect all day. 

 The observations were taken in five successive sets. 



First Set. Plane of polarisation of the pencil BF (through the 

 electric field) vertical, or perpendicular to line of force : Rise of 

 fringes indicates relative retardation of that pencil. When the 

 fringes were obtained in good form and position, the machine was 

 started, and kept working at a constant rate throughout the experi- 

 ment. As formerly, the first effect was a large disturbance, the 

 fringes being displaced and deformed, and disappearing altogether at 

 the second or third turn of the plate ; but in a little time (thirty or 

 forty turns) they reappeared in good form and approximately con- 

 stant position. For distinctness of effect the central fringe was 

 brought back to the line of reference (generally downwards) by a 

 small screw movement of the spar S ; and then, at every spark from 

 prime conductor to earth, there was a quick downward jump of the 

 fringes, the effect being as distinct as possible from the irregular and 

 slow and generally small movements that went on before and after 

 the spark. As the experiment proceeded the liquid was more 

 thoroughly mixed, the disturbance decreased, and the effect came out 

 much more purely. Sparks were taken repeatedly at every 3rd, 5th, 

 10th, 15th turn of the plate, and the jump was there in every 

 instance, and beautifully distinct. The extent of the jump varied 

 from about one-third of the fringe- width at every 5th turn of the 

 plate to about three-fourths at every 10th turn. I should add that 

 the disturbance movements, though they were greatly reduced at last, 

 were still such as to prevent any good static observation of the 

 fringes. 



It is proved clearly by this set of observations that when the 

 plane of polarisation is perpendicular to the line of force the light is 

 absolutely retarded by electric strain. The spars R and S were now 

 turned round LE through 180, and the pieces were moved across the 

 optic bench into good position. 



Second Set. Plane of polarisation of the pencil BF (through the 

 electric field) horizontal : Rise of the fringes indicates a relative 

 retardation of that pencil. The method was the same as in the first 

 set, sparks being taken from prime conductor to earth at regular 

 intervals, long and short. When the initial disturbance was over, 

 movements of the fringes were still seen, sometimes in one direction 

 and sometimes in the other, but not exclusively or specially at the 

 instant of discharge. These disturbance movements were slow and 

 generally small ; and as the experiment proceeded they became very 

 faint, and were occasionally not seen at all for a little time. As to 

 the effect specially looked for, I need only say that in several scores 



