J''. 1 On a Fundamental Question in Electro-Optics. [Mar. 15, 



experiment followed with paraffin was retained here as the best ; the 

 first internal conductor was charged by spark from the prime con- 

 ductor at regular intervals, and was put to earth for a moment in each 

 interval. 



(1.) Plane of polarisation of the pencil BP vertical : Rise of fringes 

 indicates relative retardation of BP. The fringes were generally 

 curved and very imperfect at the beginning 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 one-fifth 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 polarisation 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 incli- 

 nation, 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 poten- 

 tial, 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 ne- 

 gative. 



Second Negative Dielectric : Seal Oil. From want of homogeneity 

 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 conductor. The method of experiment was 

 the same as with oil of colza. 



(1.) Plane of polarisation of the pencil BF vertical : Rise of fringes 

 indicates relative retardation of BF. At first, the electricity pro 

 duced very large displacements and deformations of the fringes, in 



