Question in Electro-Optics. 387 



I think it must be admitted, that in this regular dislocation 

 of the fringes, there is a new and clear presentment of the 

 double refraction which is produced by electric strain. I 

 think also that the new effect is made all the more suggestive 

 by the regularity and perfect steadiness with which it comes 

 out in the midst of the disturbance. 



First appearance of the Law. — Before leaving the present 

 experiments I must notice one or two facts observed, but not 

 yet mentioned, that go towards a solution of the question 

 with which we started. The phenomena to which I refer were 

 seen clearly enough in some of the earlier experiments ; but it 

 was only at a later stage that they were well understood, and 

 they were then obtained more regularly. 



Beginning with the last form of the experiment, that with 

 the rhomb of Iceland spar as eyepiece. The spar, I should 

 mention, was always so placed that the plane of polarization 

 in the set of fringes (e) was vertical. What I have to notice 

 now is a peculiar feature of the jump of the fringes at the 

 instant of discharge. To a carefully strained, as to an un- 

 strained attention, this jump appeared as a movement of the 

 set of fringes (e) down to the level of the set (o), never as 

 a movement of the set (S) up to the level of the set (e). I 

 must say, however, that the accuracy of this perception or 

 judgment was to myself in some degree doubtful, not because 

 of any expectation that could have led to it, but because of 

 the very fugitive character of the phenomenon, and its partial 

 obscuration in many cases by disturbance. 



Returning, therefore, to the first form of experiment, that 

 with the nicol N as eyepiece. When the principal section 

 of N was horizontal, and the vibration directed therefore 

 along the line of force, there was a perfectly regular jump of 

 the fringes downwards at the instant of discharge : and at 

 high potential the effect was large and strikingly distinct. 

 When the principal section was vertical, there was nothing- 

 regular of this kind seen in any of a large number of observa- 

 tions : there were disturbance-movements at or about the 

 instant of discharge, as before and after, but nothing that 

 could be accepted as a regular jump of the fringes at that 

 instant, always in one direction or always in the other. The 

 interpretation of these results is obvious. I have already 

 stated as a matter of observation, that a rise of the fringes 

 indicates a relative retardation of the pencil BF which passes 

 through the electric field. From the downward jump of the 

 fringes in one of the two cases, we infer therefore that the 

 pencil BF is in that case relatively accelerated in consequence 

 of discharge. But in the present experiment, and with reference 

 to the pencil BF in relation to the pencil CGr, it is evident 



