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Gustav Spörer, born at Berlin in 1822, began to observe sun-spots with the view of assigning the law of solar rotation in December, 1860. Appointed observer in the new Astrophysical establishment at Potsdam in 1874, he continued his sun-spot determinations there for twenty years, and died July 7, 1895.

The period of each had lengthened by some seconds in 1883, while sudden displacements, associated with the recovery of lustre after recurrent fadings, were observed in the position of the white spot, recalling the leap forward of a reviving sun-spot. Just the opposite effect attended the rekindling of the companion object.

Before the day of the telescope, he had viewed the image of the sun as thrown on a screen in a camera-obscura, and had observed a spot on the disk which be interpreted as representing the planet Mercury, but which, as is now known, must have been a sun-spot, since the planetary disk is too small to have been revealed by this method.

It is altogether likely that it is generated in the sun, and that all the space between it and us thrills with this unknown power. All astronomers except Faye admit the connection between sun spots and the condition of the earth's magnetic elements. The parallelism between auroral and sun-spot frequency is almost perfect.

Here, then, was an action analogous to that which, as above suggested, happens around a sun-spot, where the masses of illuminated vapour constituting the photosphere are drawn towards the vortex of the cyclone, and simultaneously elongated into striæ: so forming the penumbra. At the same time there was furnished an answer to the chief objection to the cyclonic theory of solar spots.

A few years since, the astronomers who had advanced their science by aid of photography were few in number, and their results are soon enumerated. Some good pictures of the solar corona taken during solar eclipses, a series or two of sun-spot photographs, and a very limited number of successful attempts made upon the moon, and planets, and star clusters, were all the fruits of their labors.

How is it that it opposed no resistance to the motion of comets which have almost grazed the sun's surface? Is this the origin of the zodiacal light? The character of the corona in photographic records has been shown to depend upon the phase of the sun-spot period. During the sun-spot maximum the corona seems most developed over the spot-zones i.e., neither at the equator nor the poles.

Not only the penumbra but the umbra of a sun-spot, not only the umbra but the nucleus, not only the nucleus but the deeper black which seems to lie at the core of the nucleus, shine really with a lustre far exceeding that of the electric light, though by contrast with the rest of the sun's surface the penumbra looks dark, the umbra darker still, the nucleus deep black, and the core of the nucleus jet black.

The very inequalities in the sun-spot cycle are suspicious. When the sun is most spotted its total light may be reduced by one-thousandth part, although it is by no means certain that its outgiving of thermal radiations is then reduced. A loss of one-thousandth of its luminosity would correspond to a decrease of .0025 of a stellar magnitude, considering the sun as a star viewed from distant space.

Schemer thinks possible merely a photographic effect. M. Janssen considers that the photospheric cloudlets change their shape and character with the progress of the sun-spot period; but this is as yet uncertain.