512 PHILOSOPHICAL TRANSACTIONS. [anNO I768. 



happened to be -^ of an inch, and the diameter of the purple about ■^. 

 Pleased with this experiment. Dr. P. afterwards pursued and diversified it in a 

 great variety of ways ; the result of which he comprises in the following obser- 

 vations. 



1 . When a pointed piece of metal is fixed opposite to a plain surface, the 

 nearer it is placed to the surface, the sooner do the colours appear, the closer do 

 the rings succeed one another, and the less space they occupy ; as, on the other 

 hand, the farther it is placed from the surface, the later do the colours appear ; 

 but the rings then occupy a proportionably greater space, and have more room to 

 expand themselves. 



1. The more acutely pointed is the wire, or needle, from which the electric 

 matter issues, or at which it enters, the greater number of rings appear. A blunt 

 point makes the rings larger, but fewer ; and in that circumstance it is likewise 

 much later before the colours make their appearance at a given distance. 



3. In making these rings, the first appearance is a dusky red, about the edges 

 of the circular spot ; presently after which (generally after 4 or 5 strokes) there 

 appears a circular space, visible only in a position oblique to the light, and look- 

 ing like a shade on the metal. This space expands very little during the whole 

 course of the explosions, and it seems to be, as it were, an attempt at the first 

 and faintest red ; for by degrees, as the other colours fill the bulk of that space, 

 the edges of this shade deepen into a kind of brown. 



4. After a few more explosions, a second circular space is marked out by another 

 ihade, beyond the first, generally about -j- or -Jj- of an inch in diameter, which 

 was never observed to change its appearance, after ever so many explosions. 

 This second shade, by succeeding the first ; which as before observed, becomes 

 gradually of a brown, or a light red, seems to be an attempt at the fainter 

 colours, which intervene between the reds. 



5. All the stronger colours make their first appearance at the edges of the 

 circular spot ; and more explosions make them continually expand towards the 

 extremity of the space first marked out, while others succeed in their place ; till, 

 after about 30 or 40 explosions, 3 distinct rings generally appear. If the ex- 

 plosions be continued further, the circle becomes less beautiful, and less dis- 

 tinct ; the red commonly prevailing, and suffusing all the other colours. 



6. The last formed colours are always the most vivid. Also the last formed 

 rings lie closer to one another than the first. 



7. These rings may be brushed with a feather, and even wetted, or a finger may 

 be drawn over them, without their receiving any injury ; but they easily peel oft, 

 when scratched with one's nail, or any thing that is sharp, the innermost rings 

 being the most difficult to erase. 



8. The first circles are sometimes covered with a quantity of black dust ; part 



