According to my theory, a dark spot in the sun is a place in its atmosphere which happens to be free from luminous decompositions [above it]. - William Herschel

In this case, radiant heat will at least partly, if not chiefly, consist, if I may be permitted the expression, of invisible light; that is to say, of rays coming from the sun, that have such a momentum as to be unfit for vision. And admitting, as is highly probable, that the organs of sight are only adapted to receive impressions from particles of a certain momentum, it explains why the maximum of illumination should be in the middle of the refrangible rays; as those which have greater or less momenta are likely to become equally unfit for the impression of sight.

These <nowiki>[</nowiki>binary stars<nowiki>]</nowiki> may serve another very important end. ...Several stars of the first magnitude have been observed or suspected to have a proper motion; hence we may surmise that our sun, with all its planets and comets, may also have a motion towards some particular point of the heavens. ...If this surmise should have any foundation, it will show itself in a series of some years in a kind of systematical parallax, or change due to the motion of the whole solar system.

We may conceive that, perhaps in progress of time these nebulæ which are already in such a state of compression, may be still farther condensed so as actually to become stars.