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This, however, would seem to be influential only upon their white-hot or reflective ingredients, out of which the streamers and aigrettes are composed; since the coronal gases appear, from observations during eclipses, to form a shapeless envelope, with condensations above the spot-zones, or at the bases of equatorial extensions.
Each series of sun-spots is thus, to some extent, overlapped by the succeeding one; so that while the average interval from one maximum to the next is eleven years, the period of each distinct wave of agitation is twelve or fourteen. Curious evidence of the retarded character of the maximum of 1883-4 was to be found in the unusually low latitude of the spot-zones when it occurred.
As the minimum approached, the spot-zones contracted towards the equator, and there finally vanished; then, as if by a fresh impulse, spots suddenly reappeared in high latitude, and spread downwards with the development of the new phase of activity.
The ensuing period gave corresponding indications; and it may now be looked upon as established that the spot-zones close in towards the equator with the advance of each cycle, their activity culminating, as a rule, in a mean latitude of about 16°, and expiring when it is reduced to 6°. Before this happens, however, a completely new disturbance will have manifested itself some 35° north and south of the equator, and will have begun to travel over the same course as its predecessor.
When sun-spots are numerous, the corona appears to be most fully developed above the spot-zones, thus offering to our eyes a rudely quadrilateral contour. The four great luminous sheaves forming the corners of the square are made up of rays curving together from each side into "synclinal" or ogival groups, each of which may be compared to the petal of a flower.
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.
According to this view, the eclipse-halo consists of streams of matter shot out with great velocity from the spot-zones by forces acting perpendicularly to the sun's surface.
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