p 85
'Nebulous stars' must not be confounded either with irregularly-shaped
nebulous spots, properly so called, whose separate parts have an unequal
degree of brightness (and which may, perhaps, become concentrated into stars
as their circumference contracts), nor with the so-called planetary nebulae,
whose circular or slightly oval disks manifest in all their parts a
perfectly uniform degree of faint light. 'Nebulous stars' are not merely
accidental bodies projected upon a nebulous ground, but are a part of the
nebulous matter constituting one mass with the body which it surrounds. The
not unfrequently considerable magnitude of their apparent diameter, and the
remote distance from which they are revealed to us, show that both the
planetary nebulae and the nebulous stars must be of enormous dimensions.
New and ingenious considerations of the different influence exercised by
distance* on the intensity of light of a disk of appreciable diameter, and
of a single self-luminous point, render it not improbable that the planetary
nebulae are very remote nebulous stars, in which the difference between the
central body and the surrounding nebulous covering can no longer be detected
by our telescopic instruments.
[footnote] * The optical considerations relative to the difference
presented by a single luminous point, and by a disk subtending an
appreciable angle, in which the intensity of light is constant at every
distance, are explained in Arago's 'Analyse des Travaux de Sir William
Herschel' ('Annuaire du Bureau des Long.
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