of Electric Light. 371 



point, from which streams an elliptic spindle of greenish light 

 for about two-thii-ds of the distance, brightest in the axis : 

 this is full of strata, thick near the point and curved round it 

 like spheric shells, thinner below and less curved, till at the lower 

 termination of the positive light they are nearly plane and only 

 0'-05 or (y-Oe thick. In the bright central portion they seem 

 thicker than their continuations into the surrounding part, 

 which are also bent back abruptly. If the excitation be very 

 powerful, the curvature of the lower strata is reversed, these 

 being in that case formed, as Mr. Gassiot has shown, by that 

 current which passes on closing the primary circuit. Below the 

 positive light is a dark space from 0''5 to 2' wide ; and lower 

 still is an atmosphere of bright blue light (which I call the 

 aigrette). It is generally conical with a convex base, never 

 shows strata, but seems to be composed of rays diverging from 

 the negative point. From this, however, it is separated by 

 a thin dark space surrounding it like a wrapper, and within 

 that by a reddish-violet one. Along the glass tube, below this, 

 positive light with its strata reappears ; due probably to the dif- 

 ficulty of insulating the wire where the tube ends. If static 

 discharges be passed, the appearances at each point ave ^imilav — 

 a negative aigrette, a dark space, and a few thick spherical strata 

 at each end of the faint spindle of light concave towards the 

 nearest point. This is explained by the double nature of this 

 discharge. If a static spark be viewed in the mirror revolving 

 eighty times in the second, it is seen to consist of two, the 

 second narrower than the first and less lummous, about 5° 

 behind it, that is, nearly ^o'o o*^ °^ ^ second later : the two being 

 opposite in direction, produce two reverse systems, which are 

 merely superposed. 



Having premised so much, my trial of Mr. Grove's experiment 

 can be easily described. Exciting the induction-machine by two 

 Grove's, it gave static sparks = 0'-90, and dynamic = 2'-227 at 

 the rate of 7 in 2''. With the egg in circuit, no sparks passed 

 till the micrometer was at 1''038. During the whole of the 

 previous star- and brush- discharges, the appearances in the egg 

 were those which I have described as static, strata and aigrettes 

 at each end, even when the micrometer was at 4'. The mo- 

 ment a spark passed (and, as I have already said, these cannot 

 be mistaken), the strata appeared in their normal condition, 

 merely increasing in brightness and magnitude till the micro- 

 meter was in contact, liy no manipulation of it or the rhco- 

 tome could I get a spark without producing them ; nor, indeed, 

 could I get any discharge through the vacuum without getting 

 at least the static set. 



On another occasion with three Grove's, the static spark was 



