Professor Leslie on Electrical Theories . &3 
became attenuated, and hardly perceptible. We may there- 
fore demonstrably conclude, that a substance, which has the 
same electricity with the prime conductor, is disposed to emit 
light, and may infer, that a substance, which has the same elec- 
tricity with the cushion, is disposed to absorb light. 
These principles will easily explain the phenomena ot the star 
and pencil For let A (PI. I. Fig. 7.) be a pointed wire, having 
the same electricity with the prime conductor, and B another, 
having the same with that of the cushion : r ihe particles of air, 
contiguous to A, will be dispersed, as from a centre ; but their 
directions will be limited, by lines perpendicular to the surface, 
and the densest part of the current will form a cone, whose 
angle is generally about 60°, or less. As the particles proceed 
diverging, their light will continually grow more dilute, and 
melt away, by insensible degrees. Beyond CD, the luminous 
air will be so widely spread, that its glow will be totally ob- 
scured by the general lustre. Nor will the particles become 
again visible at an equal distance from B ; for since they flow 
towards that point, in all directions, they will be diffused through 
a surface EFG ; that is, at least, sixteen times larger than the 
base CD, of the cone. The particles will not, therefore, dis- 
cover their brightness, till they have approached four times 
nearer B, and, being then concentrated, they will form a lucid 
star. 
There is a circumstance, also, which will still more discrimi- 
nate the star from the pencil. It is well known, that a faintly 
illuminated body appears still darker, when viewed beside a 
bright one ; and hence a stream of fading light may be traced 
to a greater distance, if the gradations of shade are slow. But 
the rate of expansion, at CD, is sixteen times slower than at the 
equal surface OPQ ; and, consequently, though the lucid parti- 
cles are barely discernible, at the surface of the sphere, they 
may be followed to some little distance beyond the base of the 
cone. Hence, also, the pencil seems to die away, whereas the 
margin of the star is well defined. 
Perhaps the brilliancy of the aerial particles is impaired, by 
the light which they emitted, at the pencil. In that case, it 
would be necessary that the particles be still closer accumulated 
at B, before they become visible. 
