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Question in Electro- Optics* 



of an experiment ; but a few successive charges brought them, 

 after some disturbance, into permanently good form, and then 

 there was a quick downward jump seen always at the instant 

 of the spark. And, as in the contrary case of paraffin, this 

 jump was a thing as distinct as possible from the sluggish and 

 irregular disturbance-movements by which it was generally 

 followed. When the spark was taken at every 10th turn of 

 the plate, the potential was about as high as the liquid could 

 bear, and the extent of the jump was fully ^ of the fringe- 

 width. In the course of a long set of observations, this 

 downward jump of the fringes at the instant of charging was 

 seen with perfect regularity, and always distinctly. In this 

 case, therefore, the regular optical effect of electric strain was 

 an acceleration. 



(2) Plane of polarization of the pencil BF horizontal : 

 rise of fringes indicates relative retardation of BF. When 

 the fringes were imperfect at starting, the effects of a few 

 successive charges were the same as in the first case, irregular 

 displacements and changes of inclination, the fringes generally 

 rising and falling in their lower and higher parts till they 

 came into permanently good form. Afterwards there were 

 smaller disturbances always present in this case as in the 

 former ; but neither there nor here were they such as to 

 interfere ultimately with exact observation. The experiment 

 was carried on for some time till the liquid was well mixed 

 and the fringes good. Many observations were then taken, 

 some of them at highest potential ; but there was no trace of 

 a jump ever seen at the instant of the spark. In this liquid, 

 therefore, as in carbon disulphide and paraffin, the only one 

 of the two principal vibrations which is affected by electric 

 strain is that along the line of force ; but as the present 

 dielectric is of the negative class, the retardation produced is 

 negative. 



Second negative dielectric : seal oil. From want of homo- 

 geneity this liquid was very defective optically, the image of 

 the slit L being much deformed and sometimes broken by 

 streaks. The defect was remedied in a good degree by strong 

 charges given to the liquid on both sides of the second con- 

 ductor. The method of experiment was the same as with oil 

 of colza. 



(1) Plane of polarization of the pencil BF vertical: rise 

 of fringes indicates relative retardation of BF. At first, the 

 electricity produced very large displacements and deformations 

 of the fringes, in the midst of which there was no regular 

 effect to be seen ; but as the experiment went on, and the 

 medium improved, the expected effect came out distinctly, 



